At present RadFormDecorator will not decorate other RadControls (such as RadDatePicker).
For information how to produce rounded corners to the picker, please examine the following thread:
On a side note, in the future it will be easier to apply rounded corners to the RadInput control.
We are also actively researching ways to integrate the
RadFormDecorator's mechanism to be able to style RadInput and
RadDatePicker out of the box, but so far this has not been implemented.
the Telerik team