Valenti, the theme that this web site uses, stopped updating automatically at 5.5.4 and thus has not received compatibility patches for PHP8.
The only thing in the Valenti theme that is incompatible with PHP8 is a single line in a single admin function. I’ve patched this out.
You can get the patched source code (github) or download a patched zip directly.
To fix this in place for yourself, you need to edit this file:
https://github.com/chhopsky/valenti5.5-php8/blob/main/option-tree/includes/ot-functions-admin.php
Ctrl+F for create_function and replace the line with:
add_filter('option_page_capability_' . ot_options_id(), function() use ( $caps ) return $caps; }, 999);