Hi Foster,
Thanks for choosing Syncfusion products.
Sorry for the inconvenience caused. We have removed the Theme property in control. Now you can set the Theme using SkinManager.ApplyStyle(groupbar,VisualStyle.Office2003). We will update the documentation ASAP. Please use the below code for setting visual style to the control.
[XAML]
xmlns:Syncfusion="clr-namespace:Syncfusion.Silverlight.Tools.Controls;assembly=Syncfusion.Tools.Silverlight"