There is a way by which the type of your custom control can be serialized in the xml theme. You should create a class library project and implement your custom controls there. Then, you should load the types by the main menu - File >> Load Control >> From Assembly
. Please note that custom designers should be created and assigned to your custom controls. These designers will be loaded in Visual Style Builder when you want to load your control. When you save a theme, it will be saved with the desired relations. Please note that by following this approach you will lose the predefined ControlDefault theme settings.
I am attaching a sample project which demonstrates how you should implement your custom controls and the designers.
I hope this helps. If you have additional questions, feel free to contact me.
the Telerik team