Thank you for writing.
When you are using custom elements you should override the ThemeEffectiveType
property to return the type of base element - in your case RadMenuItem. For example:
This will allow your custom element to be recognized by our themes like RadMenuItem and the style for it to be applied to your custom element.
I hope this helps. Should you have any other questions, I will be glad to assist you.
All the best,
the Telerik team