If we went for a local hack (3), something like https://github.com/codebrainz/geany/commit/4d6a914c31d29a31df2a854a976305959... works, but it should be done properly as noted in the commit message if it's to actually be merged.
@apurkrt if you want to use it now, you could probably just copy the new CSS part from that commit to your `~/.config/gtk-3.0/gtk.css` file or patch `geany.css` accordingly.