It works now as follows:
1) try to get theme information from the system, and if present us it.
2) if no theme information of the system is present, take the one from
the musredit_startup.xml.
Now it is possible to switch the icon theme for the menu and toolbar
separately.
Added the needed facility in the preferences which allows to switch
the icon scheme between plain and dark. This is needed for some OS
where qt doesn't provide the necessary information.
