Add custom body class

add_filter( 'body_class', 'prefix_body_class' );
/**
 * Adds a css class to the body element.
 *
 * @param  array $classes the current body classes.
 * @return array $classes modified classes.
 */
function prefix_body_class( $classes ) {

    $classes[] = 'class-name';

    return $classes;

}