Maybe it only register as per Process, which makes sense. And it does that before any window is displayed. Actually I know when. (A bug I had) it does it in the InitCommonControlsEx call. Not even in the DLLMain. An app that needs theming needs 3 things. 1. Link to comctl32 2. call InitCommonControlsEx and 3. a manifest. Right? So I guess you have it.
Sounds plausible. Reading a bit up on window classes on MSDN (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/windowing/windowclasses/aboutwindow.asp) indicates that overrideability of the default system classes is actually intended.
I'm not 100% but I think there is no need for duplicate code. All comctl32 needs to do is chain to the user32 classes, only overriding PAINT messages and some mouse events. So you don't have to reinvent the wheel.
Sounds feasible to me.
-f.r.
