I created a test application with a treeview using implicit themes with the XAML files loaded into project. That works fine. If I move the treeview into a usercontrol in a separate project, it does not work. I tried both methods from here with no success:
It fails when loading Telerik.Windows.Controls.Navigation.xaml. It doesn't have a problem loading Telerik.Windows.Controls.xaml. The sample in the link
As a side note, I am doing the same thing with the map control and don't have any issues with it.
Link to the Project:
Here's the snippet from the UserControl: