If you want a really cool GUI look @:
http://aedgui.sourceforge.net
This one is fairly new and has a different approach as other GUI's, because it's based on SDL.
So no OS GUI API is used @ all.
Microsoft will change the GUI API completely with Blowhorn, so let them do it.
The only thing that has to be done is adapting SDL (if necessary) and the complete GUI and all GUI stuff can be as it is.
(wxWindows is different, because it uses native API stuff from the OS, the wxWindows binding for MS-Window$ needs to be completely rewritten - and this for every GUI Gadget or GUI Event - the same is valid for PureBasic's GUI Toolkit)
Sam Latinga (the guy that wrote SDL) joined the developer crew of aedgui, this tells a bit of the power of this beast.
Because it's on top of SDL it runs on every OS where you can get SDL and as long SDL runs fine on a specific OS, also is the GUI.
It's completely themable and as far as I remember you can hot switch the theme of your window.
They have a cool demo where the GUI is on ONE windowed screen with OpenGL.
Yes it's possible to mix SDL/OpenGL/GUI Gadgets on ONE screen.
The only bad thing about it is that it's written in C++.
Having access through PureBasic could be difficult.
PureBasic is Linux/Windows and supports SDL on both Operating Systems, so....