add_filter ( 'post_class', 'prefix_first_post_class' );
/**
* Add class to first post
* @param array $classes Current post classes.
* @return array Modified post classes.
*/
function prefix_first_post_class( $classes ) {
global $wp_query;
if ( 0 === $wp_query->current_post ) {
$classes[] = 'first-post';
}
return $classes;
}