Minimum required for a working UI

[whitewolf]

A friend of mine was talking about how she wants a different targeting ring, BUT doesn't like having to deal with custom UIs and keeping them updated and whatnot. So I got to thinking about it, and someone please correct me if I'm wrong, but if a custom UI doesn't have a particular file (such as the persona window) the Default one will simply be used, right?
What is the bare minimum required to be in a custom UI folder for it to function, but still HOPEFULLY use almost everything from the Default UI, and thus always be up to date?