Thanks for the interest in using our controls.
Now, straight to the problem. The only way to change the black highlight background on mouseover is to create a new control template for TopLevelHeader or TopLevelItem items. TopLevelItem is an item with RadMenu as parent and no children. TopLevelHeader is TopLevelItem with children.
At the moment Blend 2.5 doesn't support direct editing of control templates for custom controls but I believe this will change with the next version. You can find menu templates in the Telerik.Windows.Controls.Navigation assembly as resource - theme/menu.xaml. Editing this file in Blend should not be a problem. From there you can change the black background and then reuse this template in your application.
If you have any other questions, please, do not hesitate to contact us
the Telerik team
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