Qt and UI Skinning

If I understood you correctly then stylesheet is the best way forward. You can create stylesheets similar to CSS and then pass them as command line option to your application or load on invocation to style your application at runtime. That way you can create multiple stylesheets each having a different look and feel and allow user to load them at will. Since its CSS it doesn't need any new learning and you can keep all your styling outside your source code.

Here are a list of resources that can get you up and running quickly:

http://blog.qt.io/blog/2007/11/27/theming-qt-for-fun-and-profit/

http://doc.qt.io/qt-5/stylesheet.html