Along with the release of Telerik RadControls for Silverlight Beta 2, we also released a "blendable" Silverlight project. What I mean under "blendable" is that when this project is opened in Expression Blend, it allows you an easy and straightforward approach to editing the style of every control in out suite.
To begin with, go to your Client.net account and download Telerik.Windows.Controls.Blend.
Unzip the project anywhere and open the Telerik.Windows.Controls.Blend.sln files using Expression Blend.
If you explore the Project tab, you will find that each control is represented by a separate XAML file.
Open, for example, the Expander.xaml file. You will see that there is nothing more than an Expander control on the page. If you want to edit the ControlTemplate of the control, simply right-click the control and choose "Edit Control Parts(Template) > Edit Template"
After that Blend will show you all the elements that compose the ControlTemplate of the Expander control.
I am not much of a designer so I will leave the rest to your imagination.
The same instructions are fully applicable to the rest of the controls in the project. All the xaml that a single control uses is placed inside the resources of the respective host page. This means that if you want to edit the ControlTemplate of the TreeView you can find its xaml inside the UserControl.Resources tag in TreeView.xaml.