I've updated the patch to support switching between #RGBA/#RRGGBBAA and
rgba() too, fixing a bug in `css--hex-color' along the way for handling
the #RGBA case. If you agree, I think that fix should go into the
emacs-26 branch as well.

I also prefixed the function names with `css--' to more strongly
indicate that they're intended for internal usage, and mentioned in the
docstrings their incompatibility with color.el.

-- Simen