bug#25525: 25.1.90; add color highlighting to css mode

[Eli Zaretskii]

> From: Tom Tromey <tom@tromey.com>
> Cc: Tom Tromey <tom@tromey.com>,  25525@debbugs.gnu.org
> Date: Thu, 26 Jan 2017 10:13:33 -0700
> 
> I'm not sure there's a good way to use color distance to get the
> desired effect; if there were, surely Emacs core would be doing that
> already.

I thought about something like

  (if (> (color-distance color "black") 292485) "black" "white")

I think this is the equivalent of your test.

(One reason for color-distance being unused could be that it isn't
mentioned in the ELisp manual.)