.back-to-top {
  width: 3.22vw;
  height: 3.22vw;
  display: flex;
  justify-content: center;
  align-items: center;
  position: fixed;
  right: 1vw;
  bottom: 8vh;
  z-index: 8000;
  background-color: #FFF;
  box-shadow: 0 0 4px rgba(0, 0, 0, 0.14), 0 4px 8px rgba(0, 0, 0, 0.28);
  opacity: 0;
  visibility: hidden;
  transition: 300ms;
}

.back-to-top--show {
  opacity: 1;
  visibility: visible;
}

.back-to-top img {
  width: 1vw;
}

@media only screen and (max-width: 768px) {
  .back-to-top {
    width: 7vw;
    height: 7vw;
    right: 2vw;
  }
  .back-to-top img {
    width: 3vw;
  }
}