Since this reproduces only under IE it seems that your page has reached IE's 31 stylesheet limitation and further stylesheets are not loaded. More information about this browser bug is available in the following blog post: http://blogs.telerik.com/aspnetmvcteam/posts/10-05-03/internet-explorer-css-limits.aspx
which also offers a possible solution - the RadStyleSheetManager to combine the stylesheets.
the Telerik team
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