When you apply css-style, Qt will replace the object available from QAllicoation::style() on its own - QStyleSheetStyle. In the demo we're trying to get this object and cast it to the RibbonStyle. So if you plan to use the css-style you need to pre-save a reference to the RibbonStyle and use it.
I would like to clarify the purposes for which you want to use CSS?
Maybe it can be activated in the Qtitan::Office2007Stlye/RibbonStlye...