This appears to be a bug in RadEditor Prometheus, and you are right about the reason. We will fix it for the upcoming service pack of the control, scheduled for the next week. I am sorry for the inconvenience.
Meanwhile you could set the EnableEmbeddedSkin to the inner controls by finding them in the controls tree:
DialogOpener opener = RadEditor1.Controls as DialogOpener;
opener.EnableEmbeddedSkins = false;
RadWindow window = opener.Controls as RadWindow;
window.EnableEmbeddedSkins = false;
Valeri Hristov (Senior Developer, MCSD)
the Telerik team