I am using GoXAM in a Silverlight application, in combination with the Telerik Rad Controls suite for Silverlight.
I’m surprised that the Diagram’s ControlTemplate’s ScrollViewer’s scrollbars didn’t get styled automatically. Are you using Silverlight 4 and implicit styles with a style for scrollbars or for ScrollViewer?
Well, in any case, you can set the Diagram.Template to explicitly style things the way you want. The docs subdirectory of the GoSilverlight installation includes a GenericSilverlight.XAML file that shows you the standard style for Diagram, including the Setter for the ControlTemplate. Look for “DefaultDiagramTemplate”.