html {
  font: 16px Helvetica, Arial, sans-serif;
  text-align: center;
}

#display {
  margin: 2rem 0;
  font-size: 500%;
}

#button {
  background: #222;
  background: linear-gradient(to bottom, #333 0%, #111 100%);
  padding: 1rem;
  border: none;
  color: #fff;
  font: 16px Helvetica, Arial, sans-serif;
}

#button:hover {
  background: linear-gradient(to bottom, #444 0%, #111 100%);
}

#button:active {
  background: linear-gradient(to bottom, #000 0%, #222 100%);
  padding: 1.05rem 1rem 0.95rem;
}