0) If you still don't have it: install MacPorts, see http://www.macports.org for download and further instructions. You should always have MacPorts installed if you wish to run "normal" UNIX programs from the Internet. Then you have the port command at hand. if you wish you may install a full GTK subsystem if you are not content with our stripped and lean gtk4mac.
1) Install gtk-theme-switch (further details under http://blog.e-shell.org/60) by
sudo port -v install gtk-theme-switch
2) Choose some fancy themes from http://art.gnome.org/themes/gtk2. We'll choose here for example Darkilouche. You don't need to unzip and untar - just leave them stored as downloaded.
3) Launch the gtk-theme-switch by switch2 in a terminal.
4) Press the + and install a new theme, for example Darkilouche. Or try Cillop or Evil-Mac or Clearlooks-Warnings or Aero-ion3-1 (You may recall where you've stored the downloaded themes, perhaps under Downloads?). Apply.
5) Launch Z88Aurora - enjoy !