add_filter( 'body_class', function( $classes ) {
    if ( is_singular( 'job_listing' ) ) {
        global $post;
        if ( absint( get_current_user_id() ) === absint( $post->post_author ) ) {
            $classes[] = 'is-listing-author';
        }
    }
 
    return $classes;
} );