I don't know how official it is with respect to the GTK+ distribution, but I'm using the selector by Alexander Shaduri which officially belongs to a prodget called [pidgin](https://developer.pidgin.im/wiki/Using%20Pidgin#win32_gtk_theme). I don't know however to what extend pidgin is officially related to GTK. I'll copy the themes over to Geany\share\themes, as you suggested.