The best solution I can think of would be to use cPanel to delete/replace the /wp-content/themes/weaver-xtreme directory.
Some cPanels include the ability to upload a whole directory directly. Others would allow you to upload the weaver-xtreme 5.0.6 file directly, then unzip it to the themes directory.
This really is unfortunate. The bug literally involved a single space in the wrong sequence of PHP code. Totally legal PHP code except for 7.1 apparently (and perhaps others). Technically, a BIG company might be able to test PHP software on many versions, but that isn’t really possible of a small operation like Weaver Xtreme.
One also might be able to suppress display of PHP errors on the site, maybe via the DEBIG value in the wp-config.php file. Not really sure.
If one were using an auto-update package, that might also still run to update to 5.0.6.
I do know that I could reproduce the error message on one of my sites with 5.0.5, but I could still login, so there must be some level of error notification suppression that doesn’t break the login. The error seems to be issuing an error message that is breaking the http protocol for loading pages.