add_filter( 'body_class', 'custom_body_class' );
/**
 * Add custom field body class(es) to the body classes.
 *
 * It accepts values from a per-page custom field, and only outputs when viewing a singular static Page.
 *
 * @param array $classes Existing body classes.
 * @return array Amended body classes.
 */
function custom_body_class( array $classes ) {
	$new_class = is_page() ? get_post_meta( get_the_ID(), 'body_class', true ) : null;

	if ( $new_class ) {
		$classes[] = $new_class;
	}

	return $classes;
}