add = (element, classString) =>
  trackClass(
    element.classList,
    element.nodeName.toLowerCase() == "html" ? htmlClassList : docBodyClassList,
    classString.split(" ")
  )
Public inferface to add classes to the document.body.
@param {string} bodyClass The class string to be added.
                          It may contain more then one class
                          with ' ' as separator.