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

[Tom Tromey]

Simen> - It should be possible to disable it. I'm not sure whether it should
Simen>  be turned on by default or not, though I'd think that most CSS
Simen>  developers will find it useful.

I can add a defcustom.

Simen> - Selectors that happen to contain the name of a color are
Simen>  highlighted. The 'red' part of '.centered-button' is highlighted in
Simen>  red, for instance.

Thanks for noticing this.
I am sure I had \b or \< or something in the regexp earlier but I must
have dropped it.  I'll fix this up.

Tom