Hello andrea bellagamba,
It was not clear enough what version of Silverlight are you using.
I suppose that you mean Silverlight 4RC. If so then we are aware of this issue and we released an additional version of our controls made especially for SL4 RC. You can download it from here:
Unfortunately for now if you use a custom theme, you will have to replace StaticResources with their values (only in VisualStates).
Please find attached a sample project.
I hope that this will help you.
Please let us know if you have any other questions or need some more help.
the Telerik team
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