While I'll summarize below, a great tutorial on how to do this can be found here:
This solution was tested to work in OS X 10.7.3 using MacPorts version 2.0.3.
Summary of Solution:
1) install MacPorts
the project can be found at: http://www.macports.org/
2) install gtk-theme-switch in Terminal
sudo port -v install gtk-theme-switch
3) run the theme switcher app from Terminal, and use it to change themes
4) Install themes
- Some great ones can be found at http://gnome-look.org/
- You can also search MacPorts for themes (try: port search gnome-themes)
5) Additional notes
Updating themes might break some GTK-based programs. For instance, if Inkscape looks ugly rather than looking like the theme you set, it is because Inkscape only supports variants of the "Clearlooks" theme in OS X (at least for version 0.48). A solution to this would be to launch applications using their own themes, but for now this is out of the scope of this how-to.
To find out a little more on how this theme switch works, (and how to possibly do it manually), you can google up on the configuration file in your home folder named ".gtkrc-2.0".