Problem: Default color scheme is suboptimal.
Solution: Start using new color scheme. Introduce new `vim` color scheme
for opt-in backward compatibility.
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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.
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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.
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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).
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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.
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