[рони]

lieeer,

(function($){
  $(document).ready(
      function() {
      $(".demo-btn").click(function() {$("body").animatescroll()})
      $("html").niceScroll({
        cursorcolor: "#9DDAFF",
        cursorwidth: 5,
        cursorborder: "1px solid #000000",
        mousescrollstep: 50
      });
  });
})(jQuery);

<button class="btn demo-btn" onclick="$('body').animatescroll();">Click for a Demo</button>