However, since not all Windows users have local admin rights, I suggest that Geany advices the user during installation to choose an install directory where he has full control.
I know on Linux you can put themes in `~/.themes` or `~/.local/share/themes` and they get picked up, there's probably some equivalent hidden directories on Windows that GTK+ looks for themes and stuff, which doesn't require Admin permissions. A quick Google suggests one or more of these might the place to put themes:
* C:/Documents and Settings/username/Local Settings/Application Data/themes/ * C:/Documents and Settings/username/.themes/ * C:/Documents and Settings/All Users/Application Data/themes/ * C:/Documents and Settings/All Users/Documents/themes/