Nah, @codebrainz misread my reply, it was not about the amount of work (no matter how suspect @codebrainz estimates are :) but that it is unlikely to be accepted if it makes the application override the theme.
But @codebrainz reference to @b4n's plugin makes the point that there can be no objection to it being implemented as a plugin, just don't enable it if you don't like the result :)
@suntong I will however point out again that you can already control anything the GTK theme can. If you put it in the local geany.css file it will override the theme. It might take some research to find the right CSS selectors, but since that file only affects Geany you can probably be fairly blunt in your selectors.