More generally I still don't really like/understand why we need the $configdir/geany.css stuff, as the only use case for it seems to style GTK stock dialogs differently inside Geany only (which seems an artificial need, but well)
That's just an example use-case (even if the only one), but the idea is that instead of us dictating what the user can style, they can just use normal CSS without being limited by stuff we happen to have given IDs too (or stuff below it). Also, the CSS doesn't require to make every selector contain `GeanyWhatever` or `geany-whatever`.
Finally, it's a completely trivial [4-lines of code](https://github.com/geany/geany/pull/1382/files#diff-4bbd8d1b91370dfb89a7e350...) to make it work and it doesn't prevent the user from using other mechanism/file (ex. `~/.config/gtk-3.0/gtk.css`) to style Geany.
IMO it makes more sense to let user use the GTK-provided facility for that
Why can't they if they want to?