Changing this is code behind will be far away from elegant in the terms of WPF and Silverlight. However, you can inherit the RadPanelBar, override it OnApplyTemplate method and find the "arrow" with:
var arrow = this.GetTemplateChild("arrow") as Path;
If the arrow is not null then change its stroke. However, we highly encourage you to use the XAML approach.
All the best,
the Telerik team
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