I suggest that you add the CSS file in the HEAD tag of the master page (for example to App_Master/Default.master) like this:
| <title>Untitled Page</title>
| <meta http-equiv="content-type" content="text/html; charset=utf-8" />
| <link media="screen" rel="stylesheet" type="text/css" href="~/App_Themes/Blue/styles.css" runat="server" />
In the href attribute of the link tag write the path to the CSS file starting from the root folder of your site starting with ~/ . Don't forget to set the value of the runat attribute to server.
the Telerik team