GTK+ Preference Tool is an utility for easy theme and font switching for GTK+ applications. You can download the utility as a standalone source/binary in the Downloads section. It is also included in the latest GTK+ Themes packages.
Note: There may be some problems with fonts and colors when switching to/from MS-Windows theme. This seems to be a theme and/or GTK+ bug. The workaround is to restart the theme selector after switching to/from MS-Windows theme.