To my knowledge, you can’t. The backend is not “theme-able” beyond enqueuing stylesheets and JavaScript. You can make significant changes that way, but the basic structure will stay the same.
All you could do would be remove the Core pages from the menus and replace them. To the extent that you could do that, and I am not sure how far along you could get, it would be a lot of code and would probably result in a resource intensive site.