This is a little hard to do with the code editor component (Ace) that I use - I can provide token types (which become CSS class names), but I cannot provide CSS properties, so there’s no graceful way to do this without adding up to 16777216 CSS classes to cover each color (a bad idea).
I did, however, make it so that mouseovering (or putting a text cursor over if the option is enabled) a 6-digit long hex literal (0x, $) shows a little preview with a square and RGB values.
There is also support for “normal” #RRGGBB literals, which I forgot to add tooltip support for.