One code correction made for an error that appeared in debug mode. Thanks, Thad Bloom!

Add Parent Page’s Body Class to Page Descendants in WordPress

add_filter( 'body_class', 'dc_parent_body_class' );
function dc_parent_body_class( $classes ) {
    	if( is_page() ) { 
        	$parents = get_post_ancestors( get_the_ID() );
			$id = ($parents) ? $parents[count($parents)-1]: get_the_ID();
		if ($id) {
			$classes[] = 'top-parent-' . $id;
		} else {
			$classes[] = 'top-parent-' . get_the_ID();
		}
	}
 
	return $classes;
}