In order to print CSS decorated content of RadPane you need to provide path to the CSS files. In your scenario you will need to configure the RadMenu to use an external CSS files in order to be able to provide them to the print() method of RadPane.
To do so you need to follow these steps:
Set RadMenu's property EnableEmbeddedSkins to false:
For your convenience I have attached a sample project with the above mentioned implementation.
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