You can examine the rendered HTML and find that the CSS class the button has is rtClose in lightweight rendermode. It is an element of type button. This can let you build selectors strong enough to override the default settings.
Regards,
Marin Bratanov
Telerik
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 the blog feed now.