If you want to go through the hassle, there's a tutorial here:
http://classicshell.net/tutorials/skintutorial.htmlIt seems like the kind of thing that would be trivial if you understand what you're doing, but actually figuring out exactly what to do is far more difficult. A walkthrough of the exact steps might help, but I'm in no way the person to make that walkthrough, all I know myself at the moment is that the page I linked exists.
Automatic corner inversion seems like it might be a useful feature to eventually add, but there's also no guarantee that skins won't try to do something more complex with the top and bottom. Leaving the other elements alone could lead to mismatched corners, while automatically inverting them too could lead to things like upside down text elements. The only way for the system to tell what to do would be to incorporate additional flags into the skins, which could break every existing skin for only a minor cosmetic gain and still might not cover weirder cases.