 document based on dynamic changes, such as adding or removing classes.
       *
       * For performance reasons, Polymer's custom CSS property shim relies
       * on this explicit signal from the user to indicate when changes have
       * been made that affect the values of custom properties.
       *
       * @method updateStyles
       * @param {Object=} properties Properties object which is mixed into
       * the document root `customStyle` property. This argument provides a
       * shortcut for se