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

[Simen Heggestøyl]

> I just tried my patch and "$black" is not highlighted here in scss 
mode.
> I think that's expected, now that I look at it, because
> css--colors-regexp starts with a "\_<", so it only matches at symbol
> boundaries.

Hm, I'm still seeing it here with the patch from March 4. Maybe that one
is outdated?

> On second thought, I suppose checking backward one word isn't such a 
big
> deal.  I don't know SCSS though.  Is @mixin the only problem case or 
are
> there others?

I think the names of mixins and placeholders are most important, because
it's not unlikely for them to have colors for names. Examples of usage:

 @mixin black {}
 .foo { @include black; }

 %black {}
 .foo { @extend %black; }

There's also maps, but I think they're less important if it's hard to
solve:

 $foo: (bar: 1, black: 2);

-- Simen