[YITH WooCommerce Zoom Magnifier] Images appears in front of menu

This plugin causes the product image to appear in front of the main menu dropdown.

Plugin: http://wordpress.org/extend/plugins/yith-woocommerce-zoom-magnifier/

Site: http://www.loja77.com/producto/termometro-de-ouvido/45

I added this code to (functions.php) in (theme folder) but does not work.

function yit_print_custom_style() {
  ?>
  <style type="text/css">
  ul#topnav li .sub { z-index: 99999; }
  </style>
  <?php
}
add_action( 'wp_head', 'yit_print_custom_style' );