I just answered your support ticket. For your convenience I have pasted below my answer:
The CssFiles property just imports a CSS file or files in the content area of RadEditor but it does not remove the existing CSS files from it. Since you directly load the CSS files via link tags with the content through the Content property the CssFiles will not work for your scenario.
What you can do is to modify the loaded content by adding the desired CSS file according to the logged user as shown below:
"<link rel='stylesheet' href='global.css' />"
"<link rel='stylesheet' href='webcontent.css' />"
+ styles +
"</head><body><div class='globalDiv'>This div has a frame defined in global.css<br /><span class='header1'>This span och class header1 is defined in WebContent.css</span><br /><br />The drop down list should only contain header1 and NOT globalDiv</body></html>"
editor.Content = html
You can see my test in the following video: http://screencast.com/t/FAdCF7kpJ
the Telerik team
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