PHP version

Hello, I tried to use PHP version 8 (I tried 8.0, 8.1 and 8.4) but the site stoppet working, everything's back to normal when going back to back to PHP 7.4. Is it the theme that doesn't support it or should I look elsewhere? The site is: www.saintpid.se I have a test site using another theme at test.saintpid.se and that worked fine using PHP 8, that's why I suspect the theme.   Thanks!