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.

3 files changed, 108 insertions, 0 deletions

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)