I found a nice (and working on my OS, win7) tool today, a GTK theme switcher, allowing you to quickly visualize the themes you want to tweak, besides you don't need to be a tech expert to install and use it:
You'll have to download two files at http://sourceforge.net/projects/gtk-win/files/
First, get and install GTK+ Runtime Environment, go to the bin folder (on win7 = C:\Program Files (x86)\GTK2-Runtime\bin) then download GTK+ Preference Tool, extract and cutpaste the gt2_prefs.exe to C:\Program Files (x86)\GTK2-Runtime\bin, execute gtk2_prefs.exe and the theme selector launches !
Now take the themes folders you already have for gimp and place them to C:\Program Files (x86)\GTK2-Runtime\share\themes, take the engines also and place them to C:\Program Files (x86)\GTK2-Runtime\lib\gtk-2.0\2.10.0\engines
Important note: the gimp theme you have probably look like ...\themename\gtkrc but if you want them to work for the theme selector you'll have to change each one so they look like ...\themename\gtk-2.0\gtkrc.
Now, relaunch gt2_prefs.exe to see the result: