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.