My AGK2 has a text file in the Editor folder that reads...
The IDE comes with some additional themes that change the way everything in the IDE looks, these themes are located in the folder share/themes, make a note of the folder names. The currently active theme is controlled by the file data/geany.gtkrc open it and look at the last line, by default this should read
include "../share/themes/MS-Windows/gtk-2.0/gtkrc"
You can change it to another theme by using a different folder mentioned earlier, such as
include "../share/themes/ClearlooksDark/gtk-2.0/gtkrc"