Yeah isolating the theme to a set node works but has serious limitations with viewing other pages like user profiles etc.
CSS and assets aren't difficult to do, there are browser plugins that allow you to inject your own JS and CSS into a page client side, but it pages and layouts that can't be easily done.
We've setup custom pages to test new page layouts without holding stage up, so existing publishing can still go out, however it's not always possible given most builtin components need correct page context.
I'd like to see a way to setup a draft page XML and layout XML and have this render using a version=preview style parameter. As I understand it the backend does hold version history for each XML file so the data is there just need to create a way to flag the active version for normal stage access and publishing and a way to fetch draft / new versions for previewing.
