Well, installing it should make Geany work properly, but it won't change how GTK themes it too much. Still, the setting `gtk-application-prefer-dark-theme=1` works for me and selects the dark theme -- and assuming the dark theme is what you want, it should work. But I can't say whether this works at the system level, it might be overridden by the session settings or something. Try and put it in your user's config, as @codebrainz said it's indeed probably not yet existent, but you can create it.