Hi Dan-
This error is related to a new code syntax in PHP 7.4, and was patched on July 27. That said, there are a couple of ways you can fix this.
Probably the easiest way is to use your hosting control panel to revert to PHP 7.3. Once you’ve done that, update your theme and then you’ll be free to go back to the newest PHP version, 7.4.
Another way you can resolve this is by using WP-CLI via SSH. If you have SSH access to your site, use the following command:
wp theme update boldgrid-swifty
If you’re not able to use SSH or change your PHP version, another way you can resolve this is by changing your theme via phpMyAdmin.