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