The +/- buttons are meant to be transparent, showing the base_color,
but when backdropped they were picking up background-image from the base
button, meaning they suddenly became more like theme_bg_color instead,
and jumped out of the spinbutton when in backdrop unlike the rest of it.
This looks strange and achieves nothing (especially not indicating that
they are disabled, which is already served fine by their dim fg colour).

Fix this by explicitly saying we don't want any background-image there.