#back-to-top-btn {
  display: inline-block;
  background-color: #999999;
  width: 32px;
  height: 32px;
  text-align: center;
  border-radius: 4px;
  position: fixed;
  bottom: 10px;
  right: 10px;
  transition: background-color .3s, opacity .5s, visibility .5s;
  opacity: 0;
  visibility: hidden;
  z-index: 1000;
}
#back-to-top-btn::after {
  content: '\25B2';
  font-weight: bold;
  font-size: 1.2em;
  line-height: 28px;
  color: #fff;
}
#back-to-top-btn:hover {
  cursor: pointer;
  background-color: #F00;
}
#back-to-top-btn.show {
  opacity: 1;
  visibility: visible;
}
@media (min-width: 768px) {
  #back-to-top-btn {
    margin: 10px;
  }
}