Shopify keeps no file-level history of your theme. Edit a section in the theme editor, update a theme, or let two people work on the same store, and the previous version is simply gone. Auto Theme Sync keeps a complete copy of every file so you always have something to go back to.
Everything the theme contains — layout/, templates/, sections/, snippets/, assets/, config/ and locales/ — not just the files you happen to have edited recently. A 400-file theme is captured in one pass.
Files are stored by content: a file that has not changed since the last backup is stored once, not again. Backing up the same theme every day costs very little, which is what makes a real history practical. The free plan keeps 7 days, Pro keeps 90.
Pick two themes and see a colour-coded, line-by-line diff of each file, plus which files are new or identical. It answers "what actually differs between my live theme and my staging theme" in a few seconds.
Restoring a backup into a theme requires a separate approval from Shopify, on top of the theme permission granted at install. That request is with Shopify now. Backups, comparisons and history work in the meantime, and nothing in your themes is modified.
See the FAQ for how to back up a theme, how restoring works, what happens to your data on uninstall, and how storage is counted. Pricing is on the pricing page.