Take a look at this.
And if you want to get the default style to change it go to a path like this:
C:\Program Files (x86)\Telerik\RadControls for Silverlight Q3 2012\Themes\Windows8\Themes\Telerik.Windows.Controls.GridView.xaml.
Inside the xaml file you will find all the control templates. Please copy the template for the GridViewGroupRow and change it in the way you need.