When customizing Silverlight controls, there are several techniques you can follow. Each technique provides a varying level of control and development effort. If you want to change the layout of any of the built-in Silverlight controls, you will generally need to work from the existing default template contracts as a starting point. So where are these template contracts? Currently, there are two places you can find these templates:
SDK Documentation
The SDK documentation contains a subsection called Control Styles and Templates. In this section, you will find the default XAML template contracts for each control.
Generic.xaml Inside System.Windows.Controls.dll and System.Windows.Controls.Extended.dll
The default XAML templates can also be found inside the various control assemblies as an embedded resource. Simply open Reflector and navigate to the generic.xaml embedded resource.
Enjoy!