Single post content

Hi there,

you can add this PHP Snippet:

add_filter('body_class','add_category_to_single');
function add_category_to_single($classes) {
    if (is_single() ) {
        global $post;
        foreach((get_the_category($post->ID)) as $category) {
            // add category slug to the $classes array
            $classes[] = $category->category_nicename;
        }
    }
    // return the $classes array
    return $classes;
}

https://docs.generatepress.com/article/adding-php/

this will add the category slug to the single post body. You can then target it with CSS like so (add appropriate category slug):

.single-post.category-slug {
CSS Rules here
}

Yes, I have actually done that, but I would like to (need to) change the markup structure in "content-single.php" in my child theme?

So you would need to have a separate template for that particular category.
This stack exchange provides options - the responder to the OP gives the method using a filter:

https://wordpress.stackexchange.com/questions/169831/custom-single-template-for-a-specific-category