after debugging the Qt sources I found the solution.
The problem is the handling of the path in QUrl without specifying a scheme. All charachters before ':' are handled as scheme (http, file, ...).
Under Windows the path looks like "C:/RetroShare/..." and the scheme of the translated QUrl is "C". The rest after ':' is used as filename. Thats wrong.
The solution for the problem is, you have to use "file:///%style-dir%/" as path within "url()".
I checked it under Windows and Linux.