feat(highlight): update default color scheme

Problem: Default color scheme is suboptimal.

Solution: Start using new color scheme. Introduce new `vim` color scheme
for opt-in backward compatibility.

------
Main design ideas
- Be "Neovim branded".
- Be minimal for 256 colors with a bit more shades for true colors.
- Be accessible through high enough contrast ratios.
- Be suitable for dark and light backgrounds via exchange of dark and
  light palettes.

------
Palettes

- Have dark and light variants. Implemented through exporeted
  `NvimDark*` and `NvimLight*` hex colors.

- Palettes have 4 shades of grey for UI elements and 6 colors (red,
  yellow, green, cyan, blue, magenta).

- Actual values are computed procedurally in Oklch color space based on
  a handful of hyperparameters.

- Each color has a 256 colors variant with perceptually closest color.

------
Highlight groups

Use:

- Grey shades for general UI according to their design.

- Bold text for keywords (`Statement` highlight group). This is an
  important choice to increase accessibility for people with color
  deficiencies, as it doesn't rely on actual color.

- Green for strings, `DiffAdd` (as background), `DiagnosticOk`, and some
  minor text UI elements.

- Cyan as main syntax color, i.e. for function usage (`Function`
  highlight group), `DiffText`, `DiagnosticInfo`, and some minor text UI
  elements.

- Red to generally mean high user attention, i.e. errors; in particular
  for `ErrorMsg`, `DiffDelete`, `DiagnosticError`.

- Yellow very sparingly only with true colors to mean mild user
  attention, i.e. warnings. That is, `DiagnosticWarn` and `WarningMsg`.

- Blue very sparingly only with true colors as `DiagnosticHint` and some
  additional important syntax group (like `Identifier`).

- Magenta very carefully (if at all).

------
Notes

- To make tests work without relatively larege updates, each one is
  prepended with an equivalent of the call `:colorscheme vim`.

  Plus some tests which spawn new Neovim instances also now use 'vim'
  color scheme.

  In some cases tests are updated to fit new default color scheme.

24 files changed, 1150 insertions, 624 deletions

diff --git a/runtime/colors/default.vim b/runtime/colors/default.vim
index d2960fa78b..fac120ccf5 100644
--- a/runtime/colors/default.vim
+++ b/runtime/colors/default.vim
@@ -1,12 +1,10 @@
-" Vim color file
-" Maintainer:	The Vim Project <https://github.com/vim/vim>
-" Last Change:	2023 Aug 10
-" Former Maintainer:	Bram Moolenaar <Bram@vim.org>
+" Neovim color file
+" Maintainer:	The Neovim Project <https://github.com/neovim/neovim>
+" Last Change:	2023 Dec 01
 
-" This is the default color scheme.  It doesn't define the Normal
-" highlighting, it uses whatever the colors used to be.
+" This is the default color scheme.
 
-" Set 'background' back to the default.  The value can't always be estimated
+" Set 'background' back to the default. The value can't always be estimated
 " and is then guessed.
 hi clear Normal
 set bg&
diff --git a/runtime/colors/vim.vim b/runtime/colors/vim.vim
new file mode 100644
index 0000000000..3418abe424
--- /dev/null
+++ b/runtime/colors/vim.vim
@@ -0,0 +1,269 @@
+" Name:         vim
+" Description:  Vim's default color scheme
+" Author:       Bram Moolenaar <Bram@vim.org>
+" Maintainer:   The Vim Project <https://github.com/vim/vim>
+" Website:      https://github.com/vim/vim
+" License:      Same as Vim
+" Last Updated: 2023 Aug 10
+
+" This is Vim's default color scheme. It doesn't define the Normal
+" highlighting, it uses whatever the colors used to be.
+
+hi clear
+let g:colors_name = 'vim'
+
+" General
+hi Conceal      guifg=LightGrey guibg=DarkGrey guisp=NONE gui=NONE         ctermfg=LightGrey ctermbg=DarkGrey cterm=NONE
+hi Cursor       guifg=bg        guibg=fg       guisp=NONE gui=NONE         ctermfg=bg        ctermbg=fg       cterm=NONE
+hi lCursor      guifg=bg        guibg=fg       guisp=NONE gui=NONE         ctermfg=bg        ctermbg=fg       cterm=NONE
+hi DiffText     guifg=NONE      guibg=Red      guisp=NONE gui=bold         ctermfg=NONE      ctermbg=Red      cterm=bold
+hi ErrorMsg     guifg=White     guibg=Red      guisp=NONE gui=NONE         ctermfg=White     ctermbg=DarkRed  cterm=NONE
+hi IncSearch    guifg=NONE      guibg=NONE     guisp=NONE gui=reverse      ctermfg=NONE      ctermbg=NONE     cterm=reverse
+hi ModeMsg      guifg=NONE      guibg=NONE     guisp=NONE gui=bold         ctermfg=NONE      ctermbg=NONE     cterm=bold
+hi NonText      guifg=Blue      guibg=NONE     guisp=NONE gui=bold         ctermfg=Blue      ctermbg=NONE     cterm=NONE
+hi Normal       guifg=NONE      guibg=NONE     guisp=NONE gui=NONE         ctermfg=NONE      ctermbg=NONE     cterm=NONE
+hi PmenuSbar    guifg=NONE      guibg=Grey     guisp=NONE gui=NONE         ctermfg=NONE      ctermbg=Grey     cterm=NONE
+hi StatusLine   guifg=NONE      guibg=NONE     guisp=NONE gui=reverse,bold ctermfg=NONE      ctermbg=NONE     cterm=reverse,bold
+hi StatusLineNC guifg=NONE      guibg=NONE     guisp=NONE gui=reverse      ctermfg=NONE      ctermbg=NONE     cterm=reverse
+hi TabLineFill  guifg=NONE      guibg=NONE     guisp=NONE gui=reverse      ctermfg=NONE      ctermbg=NONE     cterm=reverse
+hi TabLineSel   guifg=NONE      guibg=NONE     guisp=NONE gui=bold         ctermfg=NONE      ctermbg=NONE     cterm=bold
+hi TermCursor   guifg=NONE      guibg=NONE     guisp=NONE gui=reverse      ctermfg=NONE      ctermbg=NONE     cterm=reverse
+hi WinBar       guifg=NONE      guibg=NONE     guisp=NONE gui=bold         ctermfg=NONE      ctermbg=NONE     cterm=bold
+hi WildMenu     guifg=Black     guibg=Yellow   guisp=NONE gui=NONE         ctermfg=Black     ctermbg=Yellow   cterm=NONE
+hi! link VertSplit Normal
+hi! link WinSeparator VertSplit
+hi! link WinBarNC WinBar
+hi! link EndOfBuffer NonText
+hi! link LineNrAbove LineNr
+hi! link LineNrBelow LineNr
+hi! link QuickFixLine Search
+hi! link CursorLineSign SignColumn
+hi! link CursorLineFold FoldColumn
+hi! link CurSearch Search
+hi! link PmenuKind Pmenu
+hi! link PmenuKindSel PmenuSel
+hi! link PmenuExtra Pmenu
+hi! link PmenuExtraSel PmenuSel
+hi! link Substitute Search
+hi! link Whitespace NonText
+hi! link MsgSeparator StatusLine
+hi! link NormalFloat Pmenu
+hi! link FloatBorder WinSeparator
+hi! link FloatTitle Title
+hi! link FloatFooter Title
+hi FloatShadow          guifg=NONE  guibg=Black  guisp=NONE gui=NONE    ctermfg=NONE  ctermbg=NONE   cterm=NONE blend=80
+hi FloatShadowThrough   guifg=NONE  guibg=Black  guisp=NONE gui=NONE    ctermfg=NONE  ctermbg=NONE   cterm=NONE blend=100
+hi RedrawDebugNormal    guifg=NONE  guibg=NONE   guisp=NONE gui=reverse ctermfg=NONE  ctermbg=NONE   cterm=reverse
+hi RedrawDebugClear     guifg=NONE  guibg=Yellow guisp=NONE gui=NONE    ctermfg=NONE  ctermbg=Yellow cterm=NONE
+hi RedrawDebugComposed  guifg=NONE  guibg=Green  guisp=NONE gui=NONE    ctermfg=NONE  ctermbg=Green  cterm=NONE
+hi RedrawDebugRecompose guifg=NONE  guibg=Red    guisp=NONE gui=NONE    ctermfg=NONE  ctermbg=Red    cterm=NONE
+hi Error                guifg=White guibg=Red    guisp=NONE gui=NONE    ctermfg=White ctermbg=Red    cterm=NONE term=reverse
+hi Todo                 guifg=Blue  guibg=Yellow guisp=NONE gui=NONE    ctermfg=Black ctermbg=Yellow cterm=NONE term=standout
+hi! link String Constant
+hi! link Character Constant
+hi! link Number Constant
+hi! link Boolean Constant
+hi! link Float Number
+hi! link Function Identifier
+hi! link Conditional Statement
+hi! link Repeat Statement
+hi! link Label Statement
+hi! link Operator Statement
+hi! link Keyword Statement
+hi! link Exception Statement
+hi! link Include PreProc
+hi! link Define PreProc
+hi! link Macro PreProc
+hi! link PreCondit PreProc
+hi! link StorageClass Type
+hi! link Structure Type
+hi! link Typedef Type
+hi! link Tag Special
+hi! link SpecialChar Special
+hi! link Delimiter Special
+hi! link SpecialComment Special
+hi! link Debug Special
+hi DiagnosticError          guifg=Red        guibg=NONE guisp=NONE       gui=NONE      ctermfg=1    ctermbg=NONE cterm=NONE
+hi DiagnosticWarn           guifg=Orange     guibg=NONE guisp=NONE       gui=NONE      ctermfg=3    ctermbg=NONE cterm=NONE
+hi DiagnosticInfo           guifg=LightBlue  guibg=NONE guisp=NONE       gui=NONE      ctermfg=4    ctermbg=NONE cterm=NONE
+hi DiagnosticHint           guifg=LightGrey  guibg=NONE guisp=NONE       gui=NONE      ctermfg=7    ctermbg=NONE cterm=NONE
+hi DiagnosticOk             guifg=LightGreen guibg=NONE guisp=NONE       gui=NONE      ctermfg=10   ctermbg=NONE cterm=NONE
+hi DiagnosticUnderlineError guifg=NONE       guibg=NONE guisp=Red        gui=underline ctermfg=NONE ctermbg=NONE cterm=underline
+hi DiagnosticUnderlineWarn  guifg=NONE       guibg=NONE guisp=Orange     gui=underline ctermfg=NONE ctermbg=NONE cterm=underline
+hi DiagnosticUnderlineInfo  guifg=NONE       guibg=NONE guisp=LightBlue  gui=underline ctermfg=NONE ctermbg=NONE cterm=underline
+hi DiagnosticUnderlineHint  guifg=NONE       guibg=NONE guisp=LightGrey  gui=underline ctermfg=NONE ctermbg=NONE cterm=underline
+hi DiagnosticUnderlineOk    guifg=NONE       guibg=NONE guisp=LightGreen gui=underline ctermfg=NONE ctermbg=NONE cterm=underline
+hi! link DiagnosticVirtualTextError DiagnosticError
+hi! link DiagnosticVirtualTextWarn DiagnosticWarn
+hi! link DiagnosticVirtualTextInfo DiagnosticInfo
+hi! link DiagnosticVirtualTextHint DiagnosticHint
+hi! link DiagnosticVirtualTextOk DiagnosticOk
+hi! link DiagnosticFloatingError DiagnosticError
+hi! link DiagnosticFloatingWarn DiagnosticWarn
+hi! link DiagnosticFloatingInfo DiagnosticInfo
+hi! link DiagnosticFloatingHint DiagnosticHint
+hi! link DiagnosticFloatingOk DiagnosticOk
+hi! link DiagnosticSignError DiagnosticError
+hi! link DiagnosticSignWarn DiagnosticWarn
+hi! link DiagnosticSignInfo DiagnosticInfo
+hi! link DiagnosticSignHint DiagnosticHint
+hi! link DiagnosticSignOk DiagnosticOk
+hi DiagnosticDeprecated guifg=NONE guibg=NONE guisp=Red gui=strikethrough ctermfg=NONE ctermbg=NONE cterm=strikethrough
+hi! link DiagnosticUnnecessary Comment
+hi! link LspInlayHint NonText
+hi! link SnippetTabstop Visual
+
+" Text
+hi! link @text.literal Comment
+hi! link @text.reference Identifier
+hi! link @text.title Title
+hi! link @text.uri Underlined
+hi! link @text.underline Underlined
+hi! link @text.todo Todo
+
+" Miscs
+hi! link @comment Comment
+hi! link @punctuation Delimiter
+
+" Constants
+hi! link @constant Constant
+hi! link @constant.builtin Special
+hi! link @constant.macro Define
+hi! link @define Define
+hi! link @macro Macro
+hi! link @string String
+hi! link @string.escape SpecialChar
+hi! link @string.special SpecialChar
+hi! link @character Character
+hi! link @character.special SpecialChar
+hi! link @number Number
+hi! link @boolean Boolean
+hi! link @float Float
+
+" Functions
+hi! link @function Function
+hi! link @function.builtin Special
+hi! link @function.macro Macro
+hi! link @parameter Identifier
+hi! link @method Function
+hi! link @field Identifier
+hi! link @property Identifier
+hi! link @constructor Special
+
+" Keywords
+hi! link @conditional Conditional
+hi! link @repeat Repeat
+hi! link @label Label
+hi! link @operator Operator
+hi! link @keyword Keyword
+hi! link @exception Exception
+
+hi! link @variable Identifier
+hi! link @type Type
+hi! link @type.definition Typedef
+hi! link @storageclass StorageClass
+hi! link @namespace Identifier
+hi! link @include Include
+hi! link @preproc PreProc
+hi! link @debug Debug
+hi! link @tag Tag
+
+" LSP semantic tokens
+hi! link @lsp.type.class Structure
+hi! link @lsp.type.comment Comment
+hi! link @lsp.type.decorator Function
+hi! link @lsp.type.enum Structure
+hi! link @lsp.type.enumMember Constant
+hi! link @lsp.type.function Function
+hi! link @lsp.type.interface Structure
+hi! link @lsp.type.macro Macro
+hi! link @lsp.type.method Function
+hi! link @lsp.type.namespace Structure
+hi! link @lsp.type.parameter Identifier
+hi! link @lsp.type.property Identifier
+hi! link @lsp.type.struct Structure
+hi! link @lsp.type.type Type
+hi! link @lsp.type.typeParameter TypeDef
+hi! link @lsp.type.variable Identifier
+
+if &background ==# 'light'
+  " Default colors only used with a light background.
+  hi ColorColumn  guifg=NONE      guibg=LightRed     guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=LightRed     cterm=NONE
+  hi CursorColumn guifg=NONE      guibg=Grey90       guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=LightGrey    cterm=NONE
+  hi CursorLine   guifg=NONE      guibg=Grey90       guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=NONE         cterm=underline
+  hi CursorLineNr guifg=Brown     guibg=NONE         guisp=NONE     gui=bold      ctermfg=Brown       ctermbg=NONE         cterm=underline
+  hi DiffAdd      guifg=NONE      guibg=LightBlue    guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=LightBlue    cterm=NONE
+  hi DiffChange   guifg=NONE      guibg=LightMagenta guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=LightMagenta cterm=NONE
+  hi DiffDelete   guifg=Blue      guibg=LightCyan    guisp=NONE     gui=bold      ctermfg=Blue        ctermbg=LightCyan    cterm=NONE
+  hi Directory    guifg=Blue      guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkBlue    ctermbg=NONE         cterm=NONE
+  hi FoldColumn   guifg=DarkBlue  guibg=Grey         guisp=NONE     gui=NONE      ctermfg=DarkBlue    ctermbg=Grey         cterm=NONE
+  hi Folded       guifg=DarkBlue  guibg=LightGrey    guisp=NONE     gui=NONE      ctermfg=DarkBlue    ctermbg=Grey         cterm=NONE
+  hi LineNr       guifg=Brown     guibg=NONE         guisp=NONE     gui=NONE      ctermfg=Brown       ctermbg=NONE         cterm=NONE
+  hi MatchParen   guifg=NONE      guibg=Cyan         guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=Cyan         cterm=NONE
+  hi MoreMsg      guifg=SeaGreen  guibg=NONE         guisp=NONE     gui=bold      ctermfg=DarkGreen   ctermbg=NONE         cterm=NONE
+  hi Pmenu        guifg=NONE      guibg=LightMagenta guisp=NONE     gui=NONE      ctermfg=Black       ctermbg=LightMagenta cterm=NONE
+  hi PmenuSel     guifg=NONE      guibg=Grey         guisp=NONE     gui=NONE      ctermfg=Black       ctermbg=LightGrey    cterm=NONE      blend=NONE
+  hi PmenuThumb   guifg=NONE      guibg=Black        guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=Black        cterm=NONE
+  hi Question     guifg=SeaGreen  guibg=NONE         guisp=NONE     gui=bold      ctermfg=DarkGreen   ctermbg=NONE         cterm=NONE
+  hi Search       guifg=NONE      guibg=Yellow       guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=Yellow       cterm=NONE
+  hi SignColumn   guifg=DarkBlue  guibg=Grey         guisp=NONE     gui=NONE      ctermfg=DarkBlue    ctermbg=Grey         cterm=NONE
+  hi SpecialKey   guifg=Blue      guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkBlue    ctermbg=NONE         cterm=NONE
+  hi SpellBad     guifg=NONE      guibg=NONE         guisp=Red      gui=undercurl ctermfg=NONE        ctermbg=LightRed     cterm=NONE
+  hi SpellCap     guifg=NONE      guibg=NONE         guisp=Blue     gui=undercurl ctermfg=NONE        ctermbg=LightBlue    cterm=NONE
+  hi SpellLocal   guifg=NONE      guibg=NONE         guisp=DarkCyan gui=undercurl ctermfg=NONE        ctermbg=Cyan         cterm=NONE
+  hi SpellRare    guifg=NONE      guibg=NONE         guisp=Magenta  gui=undercurl ctermfg=NONE        ctermbg=LightMagenta cterm=NONE
+  hi TabLine      guifg=NONE      guibg=LightGrey    guisp=NONE     gui=underline ctermfg=black       ctermbg=LightGrey    cterm=underline
+  hi Title        guifg=Magenta   guibg=NONE         guisp=NONE     gui=bold      ctermfg=DarkMagenta ctermbg=NONE         cterm=NONE
+  hi Visual       guifg=NONE      guibg=LightGrey    guisp=NONE     gui=NONE      ctermfg=NONE        ctermbg=NONE         cterm=NONE
+  hi WarningMsg   guifg=Red       guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkRed     ctermbg=NONE         cterm=NONE
+  hi Comment      guifg=Blue      guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkBlue    ctermbg=NONE         cterm=NONE      term=bold
+  hi Constant     guifg=Magenta   guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkRed     ctermbg=NONE         cterm=NONE      term=underline
+  hi Special      guifg=#6a5acd   guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkMagenta ctermbg=NONE         cterm=NONE      term=bold
+  hi Identifier   guifg=DarkCyan  guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkCyan    ctermbg=NONE         cterm=NONE      term=underline
+  hi Statement    guifg=Brown     guibg=NONE         guisp=NONE     gui=bold      ctermfg=Brown       ctermbg=NONE         cterm=NONE      term=bold
+  hi PreProc      guifg=#6a0dad   guibg=NONE         guisp=NONE     gui=NONE      ctermfg=DarkMagenta ctermbg=NONE         cterm=NONE      term=underline
+  hi Type         guifg=SeaGreen  guibg=NONE         guisp=NONE     gui=bold      ctermfg=DarkGreen   ctermbg=NONE         cterm=NONE      term=underline
+  hi Underlined   guifg=SlateBlue guibg=NONE         guisp=NONE     gui=underline ctermfg=DarkMagenta ctermbg=NONE         cterm=underline term=underline
+  hi Ignore       guifg=bg        guibg=NONE         guisp=NONE     gui=NONE      ctermfg=white       ctermbg=NONE         cterm=NONE      term=NONE
+else
+  " Default colors only used with a dark background.
+  hi ColorColumn  guifg=NONE     guibg=DarkRed     guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=DarkRed     cterm=NONE
+  hi CursorColumn guifg=NONE     guibg=Grey40      guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=DarkGrey    cterm=NONE
+  hi CursorLine   guifg=NONE     guibg=Grey40      guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=NONE        cterm=underline
+  hi CursorLineNr guifg=Yellow   guibg=NONE        guisp=NONE    gui=bold      ctermfg=Yellow       ctermbg=NONE        cterm=underline
+  hi DiffAdd      guifg=NONE     guibg=DarkBlue    guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=DarkBlue    cterm=NONE
+  hi DiffChange   guifg=NONE     guibg=DarkMagenta guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=DarkMagenta cterm=NONE
+  hi DiffDelete   guifg=Blue     guibg=DarkCyan    guisp=NONE    gui=bold      ctermfg=Blue         ctermbg=DarkCyan    cterm=NONE
+  hi Directory    guifg=Cyan     guibg=NONE        guisp=NONE    gui=NONE      ctermfg=LightCyan    ctermbg=NONE        cterm=NONE
+  hi FoldColumn   guifg=Cyan     guibg=Grey        guisp=NONE    gui=NONE      ctermfg=Cyan         ctermbg=DarkGrey    cterm=NONE
+  hi Folded       guifg=Cyan     guibg=DarkGrey    guisp=NONE    gui=NONE      ctermfg=Cyan         ctermbg=DarkGrey    cterm=NONE
+  hi LineNr       guifg=Yellow   guibg=NONE        guisp=NONE    gui=NONE      ctermfg=Yellow       ctermbg=NONE        cterm=NONE
+  hi MatchParen   guifg=NONE     guibg=DarkCyan    guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=DarkCyan    cterm=NONE
+  hi MoreMsg      guifg=SeaGreen guibg=NONE        guisp=NONE    gui=bold      ctermfg=LightGreen   ctermbg=NONE        cterm=NONE
+  hi Pmenu        guifg=NONE     guibg=Magenta     guisp=NONE    gui=NONE      ctermfg=Black        ctermbg=Magenta     cterm=NONE
+  hi PmenuSel     guifg=NONE     guibg=DarkGrey    guisp=NONE    gui=NONE      ctermfg=DarkGrey     ctermbg=Black       cterm=NONE      blend=NONE
+  hi PmenuThumb   guifg=NONE     guibg=White       guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=White       cterm=NONE
+  hi Question     guifg=Green    guibg=NONE        guisp=NONE    gui=bold      ctermfg=LightGreen   ctermbg=NONE        cterm=NONE
+  hi Search       guifg=Black    guibg=Yellow      guisp=NONE    gui=NONE      ctermfg=Black        ctermbg=Yellow      cterm=NONE
+  hi SignColumn   guifg=Cyan     guibg=Grey        guisp=NONE    gui=NONE      ctermfg=Cyan         ctermbg=DarkGrey    cterm=NONE
+  hi SpecialKey   guifg=Cyan     guibg=NONE        guisp=NONE    gui=NONE      ctermfg=LightBlue    ctermbg=NONE        cterm=NONE
+  hi SpellBad     guifg=NONE     guibg=NONE        guisp=Red     gui=undercurl ctermfg=NONE         ctermbg=Red         cterm=NONE
+  hi SpellCap     guifg=NONE     guibg=NONE        guisp=Blue    gui=undercurl ctermfg=NONE         ctermbg=Blue        cterm=NONE
+  hi SpellLocal   guifg=NONE     guibg=NONE        guisp=Cyan    gui=undercurl ctermfg=NONE         ctermbg=Cyan        cterm=NONE
+  hi SpellRare    guifg=NONE     guibg=NONE        guisp=Magenta gui=undercurl ctermfg=NONE         ctermbg=Magenta     cterm=NONE
+  hi TabLine      guifg=NONE     guibg=DarkGrey    guisp=NONE    gui=underline ctermfg=white        ctermbg=DarkGrey    cterm=underline
+  hi Title        guifg=Magenta  guibg=NONE        guisp=NONE    gui=bold      ctermfg=LightMagenta ctermbg=NONE        cterm=NONE
+  hi Visual       guifg=NONE     guibg=DarkGrey    guisp=NONE    gui=NONE      ctermfg=NONE         ctermbg=NONE        cterm=NONE
+  hi WarningMsg   guifg=Red      guibg=NONE        guisp=NONE    gui=NONE      ctermfg=LightRed     ctermbg=NONE        cterm=NONE
+  hi Comment      guifg=#80a0ff  guibg=NONE        guisp=NONE    gui=NONE      ctermfg=Cyan         ctermbg=NONE        cterm=NONE      term=bold
+  hi Constant     guifg=#ffa0a0  guibg=NONE        guisp=NONE    gui=NONE      ctermfg=Magenta      ctermbg=NONE        cterm=NONE      term=underline
+  hi Special      guifg=Orange   guibg=NONE        guisp=NONE    gui=NONE      ctermfg=LightRed     ctermbg=NONE        cterm=NONE      term=bold
+  hi Identifier   guifg=#40ffff  guibg=NONE        guisp=NONE    gui=NONE      ctermfg=Cyan         ctermbg=NONE        cterm=bold      term=underline
+  hi Statement    guifg=#ffff60  guibg=NONE        guisp=NONE    gui=bold      ctermfg=Yellow       ctermbg=NONE        cterm=NONE      term=bold
+  hi PreProc      guifg=#ff80ff  guibg=NONE        guisp=NONE    gui=NONE      ctermfg=LightBlue    ctermbg=NONE        cterm=NONE      term=underline
+  hi Type         guifg=#60ff60  guibg=NONE        guisp=NONE    gui=bold      ctermfg=LightGreen   ctermbg=NONE        cterm=NONE      term=underline
+  hi Underlined   guifg=#80a0ff  guibg=NONE        guisp=NONE    gui=underline ctermfg=LightBlue    ctermbg=NONE        cterm=underline term=underline
+  hi Ignore       guifg=bg       guibg=NONE        guisp=NONE    gui=NONE      ctermfg=black        ctermbg=NONE        cterm=NONE      term=NONE
+endif
+
+" vim: sw=2
diff --git a/runtime/doc/dev_theme.txt b/runtime/doc/dev_theme.txt
new file mode 100644
index 0000000000..04ec3f293c
--- /dev/null
+++ b/runtime/doc/dev_theme.txt
@@ -0,0 +1,103 @@
+*dev_theme.txt*          Nvim
+
+
+                            NVIM REFERENCE MANUAL
+
+
+Nvim theme style guide                                        *dev-theme*
+
+This is style guide for developers working on Nvim's default color scheme.
+
+License: CC-By 3.0 https://creativecommons.org/licenses/by/3.0/
+
+                                      Type |gO| to see the table of contents.
+
+==============================================================================
+Design
+
+- Be "Neovim branded", i.e. have mostly "green-blue" feel plus one or two
+  colors reserved for very occasional user attention.
+
+- Be extra minimal for 'notermguicolors' (256 colors) while allowing a bit
+  more shades when 'termguicolors' is set (true colors).
+
+- Be accessible, i.e. have high enough contrast ratio (as defined in
+  https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef).
+  This means to have value at least 7 for |hl-Normal| and 4.5 for some common
+  cases (|hl-Visual|, `Comment` with set 'cursorline', colored syntax, `Diff*`,
+  |hl-Search|).
+
+- Be suitable for dark and light backgrounds via exchange of dark and light
+  palettes.
+
+- Be usable, i.e. provide enough visual feedback for common objects.
+
+
+==============================================================================
+Palettes
+
+- There are two separate palettes: dark and light. They all contain the same
+  set of colors exported as `NvimDark*` and `NvimLight*` colors respectively.
+
+- The dark palette is used for background in the dark color scheme and for
+  foreground in the light color scheme; and vice versa. This introduces
+  recognizable visual system without too standing out.
+
+- Actual computation of palettes should be done in a perceptually uniform
+  color space. Oklch is a good choice.
+
+- Each palette has the following colors (descriptions are for dark background;
+  reverse for light one):
+
+    - Four shades of "colored" greys for general UI. In 256 colors they are
+      exact greys; in true colors they are shades of "cold" grey.
+
+        - Dark ones (from darkest to lightest) are reserved as background for
+          |hl-NormalFloat| (considered as "black"), |hl-Normal| (background),
+          |hl-CursorLine|, |hl-Visual|.
+
+        - Light ones (also from darkest to lightest) are reserved for
+          `Comment`, |hl-StatusLine|/|hl-TabLine|, |hl-Normal| (foreground),
+          and color considered as "white".
+
+- Six colors to provide enough terminal colors: red, yellow, green, cyan,
+  blue, magenta.
+  They should have (reasonably) similar lightness and chroma to make them
+  visually coherent. Lightness should be as equal to the palette's basic grey
+  (which is used for |hl-Normal|) as possible. They should have (reasonably)
+  different hues to make them visually separable.
+
+- Each palette color should have a 256 colors variant with closest color
+  computed based on the perceptually uniform distance measure.
+
+
+==============================================================================
+Highlight groups
+
+Use:
+
+- Grey shades for general UI according to their design.
+
+- Bold text for keywords (`Statement` highlight group). This is an important
+  choice to increase accessibility for people with color deficiencies, as it
+  doesn't rely on actual color.
+
+- Green for strings, |hl-DiffAdd| (as background), |hl-DiagnosticOk|, and some
+  minor text UI elements.
+
+- Cyan as main syntax color, i.e. for function usage (`Function` highlight
+  group), |hl-DiffText|, |hl-DiagnosticInfo|, and some minor text UI elements.
+
+- Red to generally mean high user attention, i.e. errors; in particular for
+  |hl-ErrorMsg|, |hl-DiffDelete|, |hl-DiagnosticError|.
+
+- Yellow very sparingly only with true colors to mean mild user attention,
+  i.e. warnings. That is, |hl-DiagnosticWarn| and |hl-WarningMsg|.
+
+- Blue very sparingly only with true colors as |hl-DiagnosticHint| and some
+  additional important syntax group (like `Identifier`).
+
+- Magenta very carefully (if at all).
+
+
+ vim:tw=78:ts=8:et:ft=help:norl:
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 825e5ba41f..f5b41f38a6 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -95,6 +95,9 @@ The following changes may require adaptations in user config or plugins.
   • Legacy and extmark signs are displayed and listed with the same priority:
       line number -> priority -> sign id -> recently placed
 
+• Default color scheme has been updated to be "Neovim branded" and accessible.
+  Use `:colorscheme vim` to revert to the old legacy color scheme.
+
 ==============================================================================
 BREAKING CHANGES IN HEAD                                    *news-breaking-dev*
 
diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index cf9b3cf0e5..f6dfe3b14a 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -30,6 +30,8 @@ Defaults					            *nvim-defaults*
   ":filetype off" to |init.vim|.
 - Syntax highlighting is enabled by default. This can be disabled by adding
   ":syntax off" to |init.vim|.
+- Default color scheme has been updated. This can be reversed by adding
+  ":colorscheme vim" to |init.vim|.
 
 - 'autoindent' is enabled
 - 'autoread' is enabled (works in all UIs, including terminal)
diff --git a/src/nvim/highlight_group.c b/src/nvim/highlight_group.c
index 3953a459bc..3bd4aa4f64 100644
--- a/src/nvim/highlight_group.c
+++ b/src/nvim/highlight_group.c
@@ -135,263 +135,321 @@ static const char e_missing_argument_str[]
 // they still work when the runtime files can't be found.
 
 static const char *highlight_init_both[] = {
-  "Conceal      ctermbg=DarkGrey ctermfg=LightGrey guibg=DarkGrey guifg=LightGrey",
-  "Cursor       guibg=fg guifg=bg",
-  "lCursor      guibg=fg guifg=bg",
-  "DiffText     cterm=bold ctermbg=Red gui=bold guibg=Red",
-  "ErrorMsg     ctermbg=DarkRed ctermfg=White guibg=Red guifg=White",
-  "IncSearch    cterm=reverse gui=reverse",
-  "ModeMsg      cterm=bold gui=bold",
-  "NonText      ctermfg=Blue gui=bold guifg=Blue",
-  "Normal       cterm=NONE gui=NONE",
-  "PmenuSbar    ctermbg=Grey guibg=Grey",
-  "StatusLine   cterm=reverse,bold gui=reverse,bold",
-  "StatusLineNC cterm=reverse gui=reverse",
-  "TabLineFill  cterm=reverse gui=reverse",
-  "TabLineSel   cterm=bold gui=bold",
-  "TermCursor   cterm=reverse gui=reverse",
-  "WinBar       cterm=bold gui=bold",
-  "WildMenu     ctermbg=Yellow ctermfg=Black guibg=Yellow guifg=Black",
-  "default link VertSplit Normal",
-  "default link WinSeparator VertSplit",
-  "default link WinBarNC WinBar",
-  "default link EndOfBuffer NonText",
-  "default link LineNrAbove LineNr",
-  "default link LineNrBelow LineNr",
-  "default link QuickFixLine Search",
-  "default link CursorLineSign SignColumn",
+  "Cursor            guibg=fg      guifg=bg",
+  "CursorLineNr      gui=bold      cterm=bold",
+  "QuickFixLine      gui=bold      cterm=bold",
+  "RedrawDebugNormal gui=reverse   cterm=reverse",
+  "TabLineSel        gui=bold      cterm=bold",
+  "TermCursor        gui=reverse   cterm=reverse",
+  "Title             gui=bold      cterm=bold",
+  "Underlined        gui=underline cterm=underline",
+  "lCursor           guibg=fg      guifg=bg",
+
+  // UI
+  "default link CurSearch      Search",
+  "default link CursorIM       Cursor",
   "default link CursorLineFold FoldColumn",
-  "default link CurSearch Search",
-  "default link PmenuKind Pmenu",
-  "default link PmenuKindSel PmenuSel",
-  "default link PmenuExtra Pmenu",
-  "default link PmenuExtraSel PmenuSel",
-  "default link Substitute Search",
-  "default link Whitespace NonText",
-  "default link MsgSeparator StatusLine",
-  "default link NormalFloat Pmenu",
-  "default link FloatBorder WinSeparator",
-  "default link FloatTitle Title",
-  "default link FloatFooter Title",
-  "default FloatShadow blend=80 guibg=Black",
-  "default FloatShadowThrough blend=100 guibg=Black",
-  "RedrawDebugNormal cterm=reverse gui=reverse",
-  "RedrawDebugClear ctermbg=Yellow guibg=Yellow",
-  "RedrawDebugComposed ctermbg=Green guibg=Green",
-  "RedrawDebugRecompose ctermbg=Red guibg=Red",
-  "Error term=reverse cterm=NONE ctermfg=White ctermbg=Red gui=NONE guifg=White guibg=Red",
-  "Todo term=standout cterm=NONE ctermfg=Black ctermbg=Yellow gui=NONE guifg=Blue guibg=Yellow",
-  "default link String Constant",
-  "default link Character Constant",
-  "default link Number Constant",
-  "default link Boolean Constant",
-  "default link Float Number",
-  "default link Function Identifier",
-  "default link Conditional Statement",
-  "default link Repeat Statement",
-  "default link Label Statement",
-  "default link Operator Statement",
-  "default link Keyword Statement",
-  "default link Exception Statement",
-  "default link Include PreProc",
-  "default link Define PreProc",
-  "default link Macro PreProc",
-  "default link PreCondit PreProc",
-  "default link StorageClass Type",
-  "default link Structure Type",
-  "default link Typedef Type",
-  "default link Tag Special",
-  "default link SpecialChar Special",
-  "default link Delimiter Special",
+  "default link CursorLineSign SignColumn",
+  "default link EndOfBuffer    NonText",
+  "default link FloatBorder    NormalFloat",
+  "default link FloatFooter    Title",
+  "default link FloatTitle     Title",
+  "default link FoldColumn     SignColumn",
+  "default link IncSearch      Search",
+  "default link LineNrAbove    LineNr",
+  "default link LineNrBelow    LineNr",
+  "default link MsgSeparator   StatusLine",
+  "default link MsgArea        NONE",
+  "default link NormalNC       NONE",
+  "default link PmenuExtra     Pmenu",
+  "default link PmenuExtraSel  PmenuSel",
+  "default link PmenuKind      Pmenu",
+  "default link PmenuKindSel   PmenuSel",
+  "default link PmenuSbar      Pmenu",
+  "default link Substitute     Search",
+  "default link TabLineFill    TabLine",
+  "default link TermCursorNC   NONE",
+  "default link VertSplit      WinSeparator",
+  "default link VisualNOS      Visual",
+  "default link Whitespace     NonText",
+  "default link WildMenu       PmenuSel",
+  "default link WinBar         StatusLine",
+  "default link WinBarNC       StatusLineNC",
+  "default link WinSeparator   Normal",
+
+  // Syntax
+  "default link Character      Constant",
+  "default link Number         Constant",
+  "default link Boolean        Constant",
+  "default link Float          Number",
+  "default link Conditional    Statement",
+  "default link Repeat         Statement",
+  "default link Label          Statement",
+  "default link Keyword        Statement",
+  "default link Exception      Statement",
+  "default link Include        PreProc",
+  "default link Define         PreProc",
+  "default link Macro          PreProc",
+  "default link PreCondit      PreProc",
+  "default link StorageClass   Type",
+  "default link Structure      Type",
+  "default link Typedef        Type",
+  "default link Tag            Special",
+  "default link SpecialChar    Special",
   "default link SpecialComment Special",
-  "default link Debug Special",
-  "default DiagnosticError ctermfg=1 guifg=Red",
-  "default DiagnosticWarn ctermfg=3 guifg=Orange",
-  "default DiagnosticInfo ctermfg=4 guifg=LightBlue",
-  "default DiagnosticHint ctermfg=7 guifg=LightGrey",
-  "default DiagnosticOk ctermfg=10 guifg=LightGreen",
-  "default DiagnosticUnderlineError cterm=underline gui=underline guisp=Red",
-  "default DiagnosticUnderlineWarn cterm=underline gui=underline guisp=Orange",
-  "default DiagnosticUnderlineInfo cterm=underline gui=underline guisp=LightBlue",
-  "default DiagnosticUnderlineHint cterm=underline gui=underline guisp=LightGrey",
-  "default DiagnosticUnderlineOk cterm=underline gui=underline guisp=LightGreen",
-  "default link DiagnosticVirtualTextError DiagnosticError",
-  "default link DiagnosticVirtualTextWarn DiagnosticWarn",
-  "default link DiagnosticVirtualTextInfo DiagnosticInfo",
-  "default link DiagnosticVirtualTextHint DiagnosticHint",
-  "default link DiagnosticVirtualTextOk DiagnosticOk",
-  "default lin