It's unclear to us how you want to use your CSS.
In Silverlight you can't use CSS to customize your application. But in RadRichTextBox you can import HTML documents which can be styled with CSS. The only requirement is the CSS to be in the HTML file and not in an external file. More about what features are supported you can read in this article: http://www.telerik.com/help/silverlight/radrichtextbox-features-html-support.html
Hope this information answers your question.