In qss, you can not use remote URLs at all!
So the url(....) syntax that is required for you to specify filenames like background-image only lets you use local filenames and resources with the :/ prefix.

This means that qrc:/ prefixes (which works fine in qml urls) does not work in qss.
This inconsistency seems like a bug to me.

Either qss should not require you to put url(...) around filenames,
or it should actually support URLs (which is probably not a good idea
for performance reasons).