Great video demonstration.

This would be a big improvement for Gutenberg and would help customizing the website layout a lot.

However, I’d suggest more work on the basis of layout for Gutenberg before going to the site-wide issue. Currently, Gutenberg has only columns layout, which is minimum. I’d like adding more options for padding, margin and responsiveness. With that, controlling the elements will be much easier. And when it comes to site-wide, it would be a game changer.