Ok, I now have two questions,
1) For curiocity purposes what would be used to write a GUI which is below GTK etc, would it be just using win32 API and x window system etc.
2) For practical purposes, what would be the best tool for creating a GUI which does not use the standard OS theme, so that the program would look the same on all OS. For example how xmms / winamp look (if they were to run on both windows and linux), I know xmms uses GTK. Or how apple's iTunes looks when running on a windows PC, I know that it basically is running a horrid kind of mac OS emulation / port of horridness.