You could do that by creating a custom CSS file and link it to your document. The context menus of the RadEditor are in a wrapper with class name reDropDownBody
. You can use this class name as parent element to style the child elements. You can find more information about the RadEdtiror's class names in this help article
I am attaching you an example project with custom stylization of the context menu, so that you could follow it as a guide for your CSS styles.
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