fix(colorscheme): use explicit normal foreground in syntax groups

Problem: Some core syntax highlight groups are cleared with intention to
  always be shown without additional highlighting. This doesn't always
  work as intended, especially with fallback mechanism of @-groups.
  Example: `Statement`/`Keyword` group shown in help code blocks
  (`@markup.raw`) is shown as bold (from `Statement`) cyan (from
  `@markup.raw`) instead of bold grey.

Solution: Explicitly use normal grey foreground in syntax groups where
  it was previously implicitly assumed.

test/functional/ui/cursor_spec.lua

@@ -248,7 +248,7 @@ describe('ui/cursor', function()
         m.attr = { background = Screen.colors.DarkGray }
       end
       if m.id_lm then
-        m.id_lm = 70
+        m.id_lm = 69
       end
     end