Problem: Updating default color scheme produced some feedback.
Solution: Address the feedback.
Outline of the changes:
- Colors `Grey1` and `Grey2` are made a little bit more extreme (dark -
darker, light - lighter) to increase overall contrast.
- `gui` colors are treated as base with `cterm` colors falling back to
using 0-15 colors which come from terminal emulator.
- Update highlight group definition to not include attribute definition
if it is intended to staty uncolored.
- Tweak some specific highlight groups.
- Add a list of Neovim specific highlight groups which are now defined
differently in a breaking way.
- Minor tweaks in several other places related to default color scheme.
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