#back-to-top {
  position: fixed;
  inset: auto 16px 16px auto; /* bottom + right */
  width: 46px;
  height: 46px;
  border-radius: 50%;
  border: none;
  background: #222;
  color: #fff;
  font-size: 22px;
  cursor: pointer;
  opacity: 0;
  visibility: hidden;
  z-index: 99999;
  transition: opacity 0.3s ease, transform 0.3s ease;
}

#back-to-top.show {
  opacity: 0.85;
  visibility: visible;
}

#back-to-top:active {
  transform: scale(0.95);
}