#back-to-top {
  position: fixed;
  bottom: 20px;
  right: 20px;
  display: none;
  z-index: 9999;
}
#back-to-top a {
  display: block;
  width: 50px;
  height: 50px;
  background-color: #333;
  color: #fff;
  text-align: center;
  line-height: 50px;
  border-radius: 50%;
  font-size: 18px;
  text-decoration: none;
}
#back-to-top a:hover {
  background-color: #555;
}