Strange, for me in Safari, it’s reporting that it refuses to execute line 315. Which is the inline script for the Discourse-spash preloader. So not theme related.
For an immediate fix, you could disable CSP using a site-setting:
Just speculating: If the issue than still persists, it might be your server environment sending something unexpected. Or another script interfering (are you running any custom plugins or other scripts?)
Note that that will leave you in a vulnerable state and should be remedied ASAP. But at least your forum is working again.