Еssentially, the Kendo UI HTML code is different and more simple than the one in the WebForms controls- it is not designed for many background images.
Currently themes like Windows 7 or Vista are not scheduled for our next releases, but I could recommend to send your idea as a feature request at our UserVoice
page. Thus way the community would be able to vote this suggestion and if it turns to be popular, we will consider its implementation.
the Telerik team