You can use the DecoratedControls option to enable the decorator to work on a specific type of control or you can use All to decorate all controls on a page. You can use it in conjunction with DecoratedZoneID to limit decoration to the controls you specify within a specific html element - useful if you want e.g. all controls within a div but nothing else on the page.
Here's a simple example showing you the answer to your specific question.
|<Telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="Buttons" skin="Sunset" />
For more information go heck out the demos at this link. FormDecorator Demo