#backtop {
  position: fixed;
  left:auto;right: 20px;top:auto;bottom: 20px;
  outline: none;
  overflow:hidden;
  color:#fff;
  text-align:center;
  background-color:rgba(49,79,96,0.84);
  height:40px;
  width:40px;
  line-height:40px;
  font-size:14px;
  border-radius:2px;
  cursor:pointer;
  transition:all 0.3s linear;
  z-index:999999;

  opacity:1;
  display:none;
}
#backtop:hover {
  background-color:#f39200;
}
#backtop.mcOut {
  opacity:0;
}