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, 12 insertions, 0 deletions

diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index 1d02e49071..6b5f0393ea 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -5136,6 +5136,18 @@ guisp={color-name}					*guisp*
 	    Black	White
 	    Orange	Purple		Violet
 
+	Colors which define Nvim's default color scheme:
+	    NvimDarkBlue    NvimLightBlue
+	    NvimDarkCyan    NvimLightCyan
+	    NvimDarkGreen   NvimLightGreen
+	    NvimDarkGrey1   NvimLightGrey1
+	    NvimDarkGrey2   NvimLightGrey2
+	    NvimDarkGrey3   NvimLightGrey3
+	    NvimDarkGrey4   NvimLightGrey4
+	    NvimDarkMagenta NvimLightMagenta
+	    NvimDarkRed     NvimLightRed
+	    NvimDarkYellow  NvimLightYellow
+
 	You can also specify a color by its RGB (red, green, blue) values.
 	The format is "#rrggbb", where
 		"rr"	is the Red value