I had a idea. It would involve a little bit code. As my original idea, we would do some basic calculations to detect light and dark themes through _GtkStyleContext_, then load _geany.css_ or _geany-dark.css_ accordingly. I think this solution is more maintainable than caring for every theme. It wouldn't add colors that are beautiful in the current theme, just colors that are easy to read. If the user wants, he would still tweak the _css_ in a similar fashion as current implemented.