#back-to-top {
  position: fixed;
  left: 24px;
  bottom: 24px;
  width: 48px;
  height: 48px;
  background: rgba(120,120,120,0.18);
  color: #444;
  border: none;
  border-radius: 50%;
  box-shadow: 0 2px 8px rgba(0,0,0,0.10);
  font-size: 2rem;
  cursor: pointer;
  z-index: 99999;
  display: none;
  align-items: center;
  justify-content: center;
  transition: background 0.2s, color 0.2s, box-shadow 0.2s;
  outline: none;
}
#back-to-top.show {
  display: flex;
  animation: backtotop-pop 0.4s;
}
#back-to-top:hover, #back-to-top:focus {
  background: rgba(44,204,113,0.18);
  color: #2ecc71;
  box-shadow: 0 4px 16px rgba(44,204,113,0.18);
}
@keyframes backtotop-pop {
  from { opacity: 0; transform: scale(0.7); }
  to { opacity: 1; transform: scale(1); }
}
@media (max-width: 600px) {
  #back-to-top {
    left: 12px;
    bottom: 12px;
    width: 40px;
    height: 40px;
    font-size: 1.3rem;
  }
}