I'm in agreement with these other fine users- there needs to be some kind of revision-history/version control built in to roll back site changes. Even something as simple as keeping server-side "snapshots" that we can restore from, that are the code files that Keith mentions. A lot of content people don't have the chops to know how to use those, so it's better for the customer to just have a button that lists snapshots with a "roll-back styles" and "roll-back all" option, so that you can revert design/layout/template changes easily, and you can also revert everything including the data.
Then before it actually executes on the action of doing so, it saves the site as-is so that if rolling-back was a bad idea then rolling-forward can also happen.
Is that a reasonable request Squarespace?