[css-fonts] [palettes] base-palette should be able to reference "dark" or "light" (just like font-palette can)

[litherum]

base-palette's current grammar is:

base-palette: <integer> | <string>

It's currently impossible to tweak the colors in the "dark" palette, because base-palette can't identify the dark palette - it can only identify palettes by their individual indices or localized name.

So, just like font-palette has:

font-palette: none | normal | light | dark | <palette-identifier>

base-palette probably should have:

base-palette: light | dark | <integer> | <string>

[litherum]

Pull request: #6679