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.

1 files changed, 5 insertions, 5 deletions

diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 2902b4a4a5..eca8ee6422 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -1185,7 +1185,7 @@ describe('float window', function()
 
     it('defaults to NormalFloat highlight and inherited options', function()
       command('set number')
-      command('hi NormalFloat guibg=#333333')
+      command('hi NormalFloat guibg=#333333 guifg=NONE')
       feed('ix<cr>y<cr><esc>gg')
       local win = meths.open_win(0, false, {relative='editor', width=20, height=4, row=4, col=10})
       if multigrid then
@@ -1271,7 +1271,7 @@ describe('float window', function()
       command('set colorcolumn=1')
       command('set cursorline')
       command('set foldcolumn=1')
-      command('hi NormalFloat guibg=#333333')
+      command('hi NormalFloat guibg=#333333 guifg=NONE')
       feed('ix<cr>y<cr><esc>gg')
       local win = meths.open_win(0, false, {relative='editor', width=20, height=4, row=4, col=10, style='minimal'})
       if multigrid then
@@ -1399,7 +1399,7 @@ describe('float window', function()
       command('set colorcolumn=1')
       command('set cursorline')
       command('set foldcolumn=1')
-      command('hi NormalFloat guibg=#333333')
+      command('hi NormalFloat guibg=#333333 guifg=NONE')
       feed('ix<cr>y<cr><esc>gg')
       local win = meths.open_win(0, false, {relative='editor', width=20, height=4, row=4, col=10, style='minimal'})
       if multigrid then
@@ -1528,7 +1528,7 @@ describe('float window', function()
       command('set cursorline')
       command('set foldcolumn=1')
       command('set statuscolumn=%l%s%C')
-      command('hi NormalFloat guibg=#333333')
+      command('hi NormalFloat guibg=#333333 guifg=NONE')
       feed('ix<cr>y<cr><esc>gg')
       meths.open_win(0, false, {relative='editor', width=20, height=4, row=4, col=10, style='minimal'})
       if multigrid then
@@ -5921,7 +5921,7 @@ describe('float window', function()
     describe('float shown after pum', function()
       local win
       before_each(function()
-        command('hi NormalFloat guibg=#333333')
+        command('hi NormalFloat guibg=#333333 guifg=NONE')
         feed('i')
         funcs.complete(1, {'aa', 'word', 'longtext'})
         if multigrid then