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.

test/functional/ui/cursor_spec.lua

@@ -213,7 +213,7 @@ describe('ui/cursor', function()
           m.hl_id = 64
           m.attr = {background = Screen.colors.DarkGray}
       end
-      if m.id_lm then m.id_lm = 69 end
+      if m.id_lm then m.id_lm = 70 end
     end
 
     -- Assert the new expectation.