Extend the body_class function

add_filter('body_class','top_level_parent_id_body_class'); function top_level_parent_id_body_class($classes) { global $wpdb, $post; if (is_page()) { if ($post->post_parent) { $ancestors=get_post_ancestors($post->ID); $root=count($ancestors)-1; $parent = $ancestors[$root]; } else { $parent = $post->ID; } $classes[] = 'top-level-parent-pageid-' . $parent; } return $classes; }


body class function