I recently installed Openbox and the Numix Gtk Theme. This looked lovely for all my Gtk3 things, like Nautilus; however, all the Gtk2 things had horrible boxy borders like this:

After some searching around, I discovered that the Gtk2 bit of Numix uses the ‘murrine engine’. So, once I installed the gtk2-engines-murrine Debian package, everything was fixed, and my Gtk2 things became beautiful.