Adds a Grand Parent class to the body

get-grandparent__grandparent_body_class.php

<?php

function get_grandparent($page_id) {
    $current_page = get_page($page_id);
    if ($current_page->post_parent > 0) {
        $parent_page = get_page($current_page->post_parent);
        if ($parent_page->post_parent > 0) {
            return $parent_page->post_parent;
        } else {
            return false;
        }
    }
    return false;
}

function grandparent_id_class($classes) {
	global $post; 
	if (is_page() && $post->post_parent && !is_front_page() && get_grandparent($post->ID)) {
		$grandparent = get_grandparent($post->ID);
		$classes[] = 'grandparent-pageid-'. $grandparent .'';
	}
	return $classes;
}
add_filter('body_class', 'grandparent_id_class');

?>