How? The way Gnome and KDE do right now, by having special themes (or theme engines) that know about the other DE's theme settings. But this is problem that exists right now as well, if you don't have those compatibility packages installed a Gnome app will look completely out of place on a KDE desktop and vice versa.
Yes those other OSes or less flexible, what does that have to do with anything? We were talking about being able to provide a consistent desktop experience, even when using CSD. And talk about insanity, you *really* want to use the Linux desktop as an example of saneness???