<div class="high">
                <button class="normal" id="myButton" value="Hover" onclick="changeClass('css/highContrast.css', 0)" ">High Contrast Mode</button>
              </div>

<script>
    function changeClass(cssFile, cssLinkIndex) {

      var oldlink = document.getElementsByTagName("link").item(cssLinkIndex);

      var newlink = document.createElement("link");
      newlink.setAttribute("rel", "stylesheet");
      newlink.setAttribute("type", "text/css");
      newlink.setAttribute("href", cssFile);
      document.getElementsByTagName("head").item(0).replaceChild(newlink, oldlink);

  document.getElementsByClassName("normal").setAttribute("text", "Normal mode");

      }
</script>