body {
  font-family: sans-serif;
  margin: 0;
  padding: 0;
}

.content {
  height: 2000px;
  padding: 20px;
}

#backToTop {
  position: fixed;
  bottom: 40px;
  right: 10px;
  width: 48px;
  height: 48px;
  display: none;
  cursor: pointer;
  opacity: 0.8;
  transition: opacity 0.0s ease;
  z-index: 999;
background-color: rgba(0, 0, 0, 0.3);
}

#backToTop:hover {
  opacity: 1;
}