I would suggest you to use our Implicit Styles. More information about them can be found in this help article
You can find the style locally, where the Telerik Ui for Silverlight is installed, inside the Themes.Implicit folder. The full path would be:
C:\Program Files (x86)\Telerik\UI for Silverlight Q1 2014\Themes.Implicit\OfficeBlack\Themes\Telerik.Windows.Controls.GridView.xaml. You can then copy the template for the GroupHeaderRow and change it per your requirements.