This is the GTK and Xfwm4 port of my IceWM theme "Waterlight," available at http://box-look.org/content/show.php/Waterlight?content=98321
As with the IceWM theme, it is minimal and intended for low light conditions. By coincidence, it also matches extremely well with Arch Linux (even though it's based on the Minimalistic Black XMMS theme).
Waterlight has something of a "*box" look to it, aided by the icon-less menus and buttons and the minimal window decorations (these turn blue on click).
The tarball contains both themes; extact it to your .themes directory with
tar -xjf WaterlightGTKXfwm4.tar.bz2 C ~/.themes or into /usr/share/themes for global access.
The inspiration for this theme is the now-dead Stygian GTK theme. It should be one of the more usable and less gaudy truly dark themes out there.