Target single post of a certain category for styling

A single post doesn’t inject the category class at the page body class by default. Here is a PHP snippet that will add the category slug to the single post body.

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;
}

You can then target it with CSS like so (add appropriate category slug).

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