i need to makes optgroups in the select based on the parent/child relationships

when i look my web at smartphone or tablet... i cant see a separation for father menu. (www.benditaflor.cl)

how can i make this visual variation?

Actually code in scripts.js is:

        // Create the dropdown base
  jQuery("<select />").appendTo("#main-menu-wrapper");

  // Create default option "Go to..."
  jQuery("<option />", {
    "selected": "selected",
    "value"   : "",
    "text"    : goText
  }).appendTo("#main-menu-wrapper select");

  // Populate dropdown with menu items
  jQuery("#main-menu a").each(function() {
    var el = jQuery(this);
    jQuery("<option />", {
      "value"   : el.attr("href"),
      "text"    : el.text()
    }).appendTo("#main-menu-wrapper select");
  });

  // To make dropdown actually work
  jQuery("#main-menu-wrapper select").change(function() {
    window.location = jQuery(this).find("option:selected").val();
  });