If I have a usercontrol (in Silverlight) that I've written, that uses XAML to define it's appearance, how can I make a customised version of it?

i.e. I have MyControl.xaml & MyControl.xaml.cs

What do I need to do if I want a "SpecialisedControl" child class? I assume I just make a new code file, then inherit from MyControl. But what if I want to change the appearance of the base class - then what do I do?