Actually, nowadays that is possible to do for xmlc skins also.
You can create an xml file, only containing the element(s) you want changed from an existing xmlc skin.
Reference the name of that source skin in the xml file, make the required changes, rename the xml file to your liking, and copy it to the skin folder containing the source xmlc file.
You can then select that new skin file in MusicBee.
Here is the
download for an example using the default MusicBee3 skin as a source, only changing the 'no albumart' image.