Support currentColor in font color palettes
Maybe there are other keywords in `<color>` which need to be handled too?
<rdar://problem/83343497>
Looks like color keywords are idents, otherwise it's a color primitive value.
color functions 🤔
See: https://github.com/w3c/csswg-drafts/issues/6680
Closing as MOVED until https://github.com/w3c/csswg-drafts/issues/6680 is resolved.