.back-to-top {
  display: flex;
  justify-content: center;
  align-items: center;
  z-index: 2;
  border-radius: 50%;
  cursor: pointer;
  width: 60px;
  height: 60px;
  position: fixed;
  opacity: 0;
  transform: translateY(100px);
  transition: all .5s ease
}
    
.back-to-top__show {
  opacity: 1;
  transform: translateY(0)
}