Can I download a copy of my current theme as a backup file?

Yes. On Settings → Appearance → Themes, click Export Theme at the top right. Wcart downloads your active theme as a .json file named after the theme, with its images included. If some content could not be captured, you'll see a warning saying how many items were skipped.

The Themes page has no button to upload that file again, so keep it as an archive copy. For a backup you can restore in one click, use Save & Activate when you switch themes. Your current design is then kept under Your Saved Themes on the Installed tab, and you can click Activate on it later to bring it back.