Since Gtk 3.20 all/most external themes are broken except Adwaita, HighContrast and Raleigh.
What's failed: no buttons or menus are drawn.
In Synaptic the toolbar buttons disappeared in Adwaita, too.
Perhaps I find a patch which fixes or disarm this issue: https://bugs.freedesktop.org/show_bug.cgi?id=94887
I will give it a shot to see what's happen ...
- Thomas --