Steam version, or standalone?
Steam version:
1) Right click on AppGameKit in the apps list in steam and open Preferences.
2) Go to "Local Files" tab.
3) Click on "Browse Local Files..." button.
4) Right click on the AGK2 app in Finder and "Show Package Contents".
5) Navigate to "Contents/Resources/etc/gtk-2.0/" and open the "gtkrc" file in a text editor.
6) Edit the first line: include "......." to be the path to the gtkrc file for the theme you want to use. (They are in the AGK2 app package.)
So for mine I have it using the dark theme, so my line reads: include "../../share/themes/ClearlooksDark/gtk-2.0/gtkrc"
(You can probably just edit that "ClearlooksDark" part to whatever theme you want to use, provided it is installed in the same folder as the default themes that are included.)
Standalone:
For the non-Steam version, you do the same as above, but you have to locate the AGK2 app wherever you installed it instead of in the Steam apps folder.
Hope that helps!
Using AppGameKit v2 T1 + T2!
Systems: Primary: Mac OS X 10.10
Secondary: Windows 7