I finally got tired of the default gtk theme on my macbook, so I decided to do something about it. I've been using gtk2 no_x11 variant provided by macports, but unfortunately all the theme engines on macports require X11. I tried the GtkQuartzEngine, and it seems to work perfectly with a little tweaking.
1. Check out git://github.com/jralls/gtk-quartz-engine.git
2. Cd into the gtk-quartz-engine repo and run aclocal && autoreconf -vi && autoconf (I used the macports versions)
3. ./configure --prefix=/opt/local && make && make install
4. Copy the sample gtkrc file to ~/.gtkrc-2.0.
And it all just worked. It looks decent, albeit slightly buggy. It's necessary to set the prefix to /opt/local so the engine is installed into the same location as the macports gtk libs.
This seems like a relatively simple candidate for adding to macports.