The RadMenuItem uses a ButtonChrome control that is responsible for visual states. To change one of its state , you need to edit the Style for the ButtonChrome and apply the new Style to the ButtonChrome inside the RadMenuitem template.
The information in the article is a bit outdated, for which I hope you can accept our apologies. We will update that described approach as soon as possible.
For now, I prepared a very basic sample that has a changed mouse over state for both the top level item and submenu item. Some useful information on the topic can be also found here
I hope this helps.
the Telerik team
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