Ok so you are on Windows and you want to access the underlying WPF…
I’m sure there is a hack possible to do that. However, if you are on Windows, I would just create a custom WPF window in Visual Studio, put it into a custom plugin or into a plain .dll (loaded during runtime) and then just call this window in Rhino by doing:
// Create a custom WPF window with the VS Designer using the XAML markup
MyCustomView window = new MyCustomView();
// (if MVVM, inject the viewmodel)
window.DataContext = new MyCustomViewModel();
// open the window, use .ShowDialog() for a different behaviour
window.Show ();
With this approach, you can make use of the powerful Visual Studio Designer, which also allows you to copy and override the style of any user control. See MSDN documentation pages on how to do that.
See my post from above regarding the ComboBox. But at first this is overwhelming and difficult.
So I would also suggest buying a book about WPF and just learn how this works. It has a steep learning curve, but it’s very powerful if you understand this technology, especially in combination with Visual Studio! Once you know it’s easy to create custom controls with advanced designs.