When the window was backdropped, they suddenly regained their border.
This was clearly not intentional or of any practical use to anyone.

Shuffle around some selectors so that the backdrop ones do not override
the flat ones and make the borders magically reappear when backdropped.

Note that, whereas standard titlebuttons get the border on :hover, other
.flat buttons in the headerbar do not. That should probably be fixed too

https://bugzilla.gnome.org/show_bug.cgi?id=788580