JFTR: beginning with Geany 1.38, the Windows builds use GTK3 and here is what is needed:
Create or edit the file `C:\Users<username>AppData\Local\gtk-3.0\settings.ini` with the following content: ```ini [Settings] gtk-theme-name=Windows10 ```
And your theme files should be located in `C:\Users<username>\AppData\Local\themes\Windows10`. Replace "<username>" with your username and "Windows10" with the theme you want to use.