bug#30295: CSS mode colour highlighting makes text hard to read

[Richard Copley <rcopley@gmail.com>]

In CSS mode, the value "green" or "#0f0" or "rgba(0,255,0,0.5)" is
displayed with a white foreground and a 100%-green background, which
is difficult to see.

A white foreground is chosen on even lighter colours, for example,
"#5e5" and "#87e087".

See functions css--contrasty-color in textmodes/css-mode.el and
color-distance in src/xfaces.c, and
<https://debbugs.gnu.org/cgi/bugreport.cgi?bug=25525>, and
<https://www.compuphase.com/cmetric.htm>.

If the color-distance is to be used, the appropriate formula is:

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

That formula still maps "green" to "white", but it does at least map
lighter colours such as "#38e038" to "black".

For what it's worth, in my inexpert opinion it makes more sense to
compare the luma against that of mid-grey as Tom originally proposed.