Enable support for CSS variables in color property value on docs.json

[dgonzalezr]

Currently, the docs.json only allows color values that match the pattern of ^#([a-fA-F0-9]{6}|[a-fA-F0-9]{3})$.

It will be good if the regexp can be extended so that it also supports CSS variable strings as values. That could give the user more flexibility when creating custom styles for the documentation.