I believe that you are mistaken. The theme works just fine with PHP 8.0+. The theme doesn’t include any custom widgets in order to call the WP_Widget class. Are you using version 1.3.1? Have you tried disabling plugins?
Thank you for fast reply, Dumitru. And you were right! I turned on detailed error log and found plugin causing error (Scrollorama). Now it’s disabled and site works OK, thank you!