In reply to Fränk.

Adding to this a bit. In my mind, I see WP itself or either a plugin giving end-users the ability to create a full theme ZIP package. Then, they could share it with friends or even do something like upload it to the official theme directory.

This is actually a bit more complex than what the site editor can currently do. There are things like copyright and licensing to take into account -- we really need a standard here. The theme.json file would be needed. There are some other files like the style.css and index.php that would need to be included (as far as I know, the theme system still requires these). Of course, there'd need to be a screenshot.

I really think this starts getting into plugin territory, but the block system would greatly simplify the whole thing.