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, 19 insertions, 7 deletions

diff --git a/test/functional/ui/screen_basic_spec.lua b/test/functional/ui/screen_basic_spec.lua
index 7cc1accd3f..4ef7565ec5 100644
--- a/test/functional/ui/screen_basic_spec.lua
+++ b/test/functional/ui/screen_basic_spec.lua
@@ -8,9 +8,15 @@ local funcs, meths = helpers.funcs, helpers.meths
 
 describe('screen', function()
   local screen
-  local nvim_argv = {helpers.nvim_prog, '-u', 'NONE', '-i', 'NONE', '-N',
-                     '--cmd', 'set shortmess+=I background=light noswapfile belloff= noshowcmd noruler',
-                     '--embed'}
+  local nvim_argv = {
+    helpers.nvim_prog,
+    '-u', 'NONE',
+    '-i', 'NONE',
+    '-N',
+    '--cmd', 'set shortmess+=I background=light noswapfile belloff= noshowcmd noruler',
+    '--cmd', 'colorscheme vim',
+    '--embed',
+  }
 
   before_each(function()
     local screen_nvim = spawn(nvim_argv)
@@ -997,9 +1003,15 @@ describe('Screen default colors', function()
   local function startup(light, termcolors)
     local extra = (light and ' background=light') or ''
 
-    local nvim_argv = {helpers.nvim_prog, '-u', 'NONE', '-i', 'NONE', '-N',
-                       '--cmd', 'set shortmess+=I noswapfile belloff= noshowcmd noruler'..extra,
-                       '--embed'}
+    local nvim_argv = {
+      helpers.nvim_prog,
+      '-u', 'NONE',
+      '-i', 'NONE',
+      '-N',
+      '--cmd', 'set shortmess+=I noswapfile belloff= noshowcmd noruler'..extra,
+      '--cmd', 'colorscheme vim',
+      '--embed',
+    }
     local screen_nvim = spawn(nvim_argv)
     set_session(screen_nvim)
     screen = Screen.new()
@@ -1017,7 +1029,7 @@ describe('Screen default colors', function()
   it('can be set to light', function()
     startup(true, false)
     screen:expect{condition=function()
-      eq({rgb_fg=Screen.colors.White, rgb_bg=0, rgb_sp=Screen.colors.Red,
+      eq({rgb_bg=Screen.colors.White, rgb_fg=0, rgb_sp=Screen.colors.Red,
           cterm_bg=0, cterm_fg=0}, screen.default_colors)
     end}
   end)