Posting back in hoping that this can be expedited, as I found myself stuck in such situation.
- I'm using Ubuntu Mate 18.04.1, and - I found myself not able to switch the GTK theme -- flipping through the Themes in `gnome-tweak-tool`'s "Applications" drop down has no effects at all. - and now, I found myself can't even switch geany-theme either -- I chose a light theme (from https://github.com/codebrainz/geany-themes/tree/master/screenshots#the-epsil...) by selecting Change Color Scheme... from Geany's View menu, _restarted_ `geany`, but the editor still has a dark window:
![image](https://user-images.githubusercontent.com/422244/59976616-11095200-9595-11e9...)
Thanks!