Found the solution. For those with similar issues, please follow this instruction.
https://www.wordfence.com/help/advanced/remove-or-reset/
Hi @mmmolst, thanks for posting the solution you found for others searching these terms.
Wordfence should be in Learning Mode for the first seven days after installation, but it sounds like this could have helped for allowing the editing of your theme. Should you wish to reinstall Wordfence and try again, let me know in a new topic any details surrounding the exact steps you took to receive the 403 error, and specifics of any error messages you see crop up. You can include screenshots if necessary by using a service such as Snipboard.
Thanks,
Peter.