GTK 3.20 doesn't like getting a style property for a non-current state,
unless the call is wrapped in a save()/restore() pair.

So, fix the code to either use the current state or temporarily save
the context.

gb.c now uses different states, but it shouldn't really matter given
how they are used, and even gives a native behavior when the window
loses focus, as it now properly reacts to BACKDROP state.