Basic lists a weekly backup target. Professional and Unlimited list a daily target. A target is not a guarantee that today's upload is already in the copy. Before you change a theme, export your own copy.
Store that copy off the account. A backup that sits only on the same disk does not help if the account is the thing that breaks. The return policy is explicit that backups are for active accounts and are not an archive after a term is closed.
Restore drills matter more than the schedule. Once a term, download a backup and see that it opens. A file you have never opened is a hope, not a backup.
If a plugin update blanks the site, the outage credit does not apply. The credit is for a fault on the platform, not for application code. The terms page says the same thing.