I modify the old gtk2_prefs.exe (GTK theme selector) to only able to modify host application's gtk (a.k.a per apps setting), for example if you put this on inkscape installation you can only modify theme for inkscape and wont affect other (e.g Gimp). This is great for portable apps? or admin user who have privilege over %programfiles%.
The original version defaulted to current user setting which effectively render all GTK based apps you're installed to obey it. If you already apply this setting you must delete %USERPROFILE%\.gtkrc-2.0 (note: this is a file not folder).
gtk2_prefs.exe (place this along side main executable e.g gimp.exe, inkscape.exe, mypaint.exe)
Refer to this post to add some themes