body {
  font-family: Arial, sans-serif;
}

#checkbox:checked {
  background-color: #6B7280;
  border-color: #888;
}

#checkbox:checked::after {
  content: '';
  position: absolute;
  top: 0px;
  left: 5px;
  width: 6px;
  height: 12px;
  border: solid white;
  border-width: 0 2px 2px 0;
  transform: rotate(45deg);
}

@keyframes fadeOut {
  from {
    opacity: 1;
  }
  to {
    opacity: 0;
  }
}

.fade-out {
  animation: fadeOut 0.5s forwards;
}