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.
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