I would be interested in contributing to this.
I would disagree that the GUI library should be restricted to native widgets, though. There is a reason that Qt is the defacto standard when it comes to cross-platform GUI development, and the reason a lot of high-end GUI applications use it.
The style system isn't just a toy for viewing applications in Motif style on Windows, there are many practical uses for it. It allows you to create a custom and consistent look-and-feel for an entire application or suite of applications, in as fine-grained a manner as needed, without having to intersperse a bunch of look-and-feel rendering code throughout all of your widgets. Consider an application like Autodesk Maya, and imagine how tedious it would be to create all of those little widgets and custom list/tree controls in a consistent manner across platforms by using native controls.
For example, if the GUI library uses only native controls, and you want to create a list or table control that varies significantly from the native list-view controls, you would essentially need to recreate all of the logic for a list-view control yourself, as well as all of the rendering. If you want to make some similar changes to a tree control, you would have to do the same thing for tree controls. Whereas with Qt's style system, you could just go into the style class and modify the code that draws the scroll bar, or the header elements in a multi-column table, and that same code will work on all platforms, and it will work for all similar controls that use that style.
There are also lots of strange quirks and bugs in different native GUI systems that make advanced cross-platform GUI development with native widgets a monstrosity. Try supporting some advanced behavior with the Windows ListView control sometime and see if your head doesn't explode
This is not to say that a library shouldn't use native widgets. Ideally I think you would want something like a basic GUI system that uses native controls for simple applications with simple GUI requirements, and an advanced GUI system that uses a system of custom rendering for most/all common controls. This would allow for applications that have simple GUI requirements to avoid the overhead of an advanced GUI system, but also allow a more productive way to develop advanced GUI systems.