Hello Christian,
You could modify the RadMenuItemStyle and set different control tempaltes for TopLevelItems. The properties you need to modifies are listed in the code snippet below:
Inside the Custom control template replace Border named HighlightVisual with the ButtonChrome button. Keep in mind you need to modify the VisualStates to match the desired scenario.
Hope it helps.
Regards,
Masha
Telerik
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