However, adding the following CSS code to
[[MediaWiki:Common.css]] had
no effect whatsoever [...]
I've realized that I need to use
#ca-edit a { ... }
to apply the background image.
However, I now have the problem that padding (or any attempt to change
the button's width) is ignored, so the button's label text overlaps with
the icon.
Unfortunately, I have not been able to find a workaround for this, so
I'd greatly appreciate some assistance!
-- F.