qss urls are not really urls, but filenames

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).