It’s not a theme bug, it’s your server memory exhaust.
Meaning there is not enough memory on your server to run your website properly. You need to contact your web hosting company, you probably need to pay some fees for increasing memory.
Please note that defining WP_MEMORY_LIMIT with a greater value will not work unless you have “spare” memory allocated. The best way is to contact your web hosting company.
The following is the error message I saw in your webpage source code.
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 9126605 bytes) in /datas/vol2/w4a136058/var/www/laclassebleue.fr/htdocs/wp-includes/wp-db.php on line 1092