попробуйте
/**
* Adds custom classes to the array of body classes.
*
* @param array $classes Classes for the body element.
* @return array
*/
function White_theme_body_classes( $classes ) {
// Adds a class of group-blog to blogs with more than 1 published author.
if ( is_multi_author() ) {
$classes[] = 'group-blog';
}
// Adds a class of hfeed to non-singular pages.
if ( ! is_singular() ) {
$classes[] = 'hfeed';
}
return $classes;
}
add_filter( 'body_class', 'White_theme_body_classes' );
и в header.php добавьте этот код
<body <?php body_class(); ?>>