Custom class output depending on category

Sorry for the delayed response.

Check this filter out:
https://github.com/WebberZone/contextual-related-posts/blob/v3.0.7/includes/main-query.php#L126

You will need to create a function to filter crp_post_class to add the additional classes you want.

Thanks a lot.
Can you give me an example of what such a function might look like? I’m not the very best coder :/

Something along these lines:

add_filter('crp_post_class','more_crp_classes');
function more_crp_classes( $post_classes ) {
    $post_classes .= ' new_class ';
}

add_filter('crp_post_class','more_crp_classes');
function more_crp_classes( $post_classes ) {
    $post_classes .= ' new_class ';
    return $post_classes;
}

But…
Seems not the aim to add a class to the main container, but to every post item, right?

This is the wrapper to the div:

https://github.com/WebberZone/contextual-related-posts/blob/a8cdd3949bfdcfde24a19b5c35a06bf2a75572b5/includes/main-query.php#L126