feat(highlight): tweak default color scheme

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.

1 files changed, 4 insertions, 2 deletions

diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index 220505f573..e0ad0ff0f9 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -30,8 +30,10 @@ 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|.
+- Default color scheme has been updated. This can result in color schemes
+  looking differently due to them relying on how highlight groups are defined
+  by default. Add ":colorscheme vim" to |init.vim| or your color scheme file to
+  restore the old default links and colors.
 
 - 'autoindent' is enabled
 - 'autoread' is enabled (works in all UIs, including terminal)