All credit for the awesome GTK theme goes to corenominal. It was just a starting point for my Openbox theme because it is such a great looking theme.
I made a different Openbox theme (before I knew there was already an openbox theme), with the intent of the window titles and borders integrating better with the windows. The GTK theme also works great with the Rezlooks GTK engine, if you edit the engine parameter in the gtkrc from clearlooks to rezlooks.
Here's the original by Corenominal: http://box-look.org/content/show.php/Darkness+Returns+GTK%2B+%26+Openbox+themes?content=80903