MYTHEME.script.js

(function ($) {
  Drupal.behaviors.menuTree = {
    attach: function (context, settings) {
      // Set ul depths for better theming.
      $('#main-menu ul').each(function () {
        var depth = $(this).parents('ul').length;
        $(this).addClass('ul-depth-' + depth);
      });

      // Set ul > li depths for better theming.
      $('#main-menu ul li').each(function () {
        var depth = $(this).parents('li').length;
        $(this).addClass('li-depth-' + depth);
      });

      // Set li > a depths for better theming.
      $('#main-menu ul li a').each(function () {
        var depth = $(this).parents('ul').length;
        $(this).addClass('ula-depth-' + depth);
      });
    }
  }
});