#backtotop {
  z-index: 999;
  display: inline-block;
  position: fixed;
  /* visibility: hidden; */
  bottom: 20px;
  right: 20px;
  width: 36px;
  height: 36px;
  line-height: 36px;
  font-size: 16px;
  text-align: center;
  color: #ffffff;
  background-color: #8ec837;
  opacity: 0.2;
}
#backtotop i {
  display: block;
  width: 100%;
  height: 100%;
  line-height: inherit;
}
#backtotop.visible {
  visibility: visible;
  opacity: 0.8;
}
#backtotop:hover {
  opacity: 1;
}