Unfortunately RadScheduler does not provide such a customization of its advanced form. You are able to translate
the user interface, but the controls does not provide a way to change the the overall appearance out-of-the-box.
You could, however, create your own advanced form and style the elements in a way it suits your needs best. Here are a couple of links which show more on the matter:
I hope this will help.
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