Its possible to do this with luxrender, we use Qt for our GUI and it does support CSS based style sheets however discussion of this tends to bring out the debate about the value of UI skins given many or even most people want the GUI to match their OS definition vs the idea that different skins would help with integration and also viewing images, there are various technical reasons related to perception of color for example that suggest that dark gray might be the best color scheme for luxrender.
I have researched implementing style sheets in luxrender as loadable files but never moved forward with it for both the above reasons, but also the fact that luxrender has a full screen mode and it could get very tedious to implement new skins because of all the little bugs that would slip thru the cracks even tho it would be simple initially.
Its always on the table though
