There is no option to tell where to output style sheets added in the markup in relation to built-in style sheets. Since they are in the markup they are the first style sheet references, which exist in the style sheet collection; RadControls' and any other embedded style sheets are added at a later stage and so they appear after the ones in the markup.
Currently, you can explicitly add the style sheets of a control before the one in the markup. Of course, if you have lots of controls this will be a tedious operation, however it is the only way.
I hope this helps.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed now