I agree with Imacken... Anyway, to answer you question, themes should be under /Extra/Themes/. If this Themes folder doesnt exist, I would be surprised if you had a GUI at all? (or maybe on is coded straight into cham). Anyway each theme should have its own folder, and the theme you want should be specified in boot.apple.com.plist (if you don't know how to do this, use search). To be completely sure, for example if you go to /Extra/Themes/Any Theme/, you should see all the theme files directly inside.