Even if this thread is closed and a bit old already, I want to notify, that at least I also and still (mid 2017) would greatly welcome a nice dark theme for Geany. There is one example, who ignores the global settings and goes dark entirely, with a modern GTK3 look in addition, namely gThumb (the photo and image viewer, in my case ver. 3.5.2).
By the way, how does it go with GTK3? I ask, because many other issues like enhancements could be easier to do and fixed within GTK3, besides the theming problem.