html, body {
  margin: 0;
  padding: 0;
  height: 100%;
}

body {
  background-color: white;
  background-size: cover;
  background-repeat: no-repeat;
  background-position: center center;
  transition: background-image 0.5s ease;
}

#doorbell-knob {
  width: 150px;
  height: 150px;
  margin: 100px auto;
  background-image: url('knob.jpg');
  background-size: contain;
  background-repeat: no-repeat;
  background-position: center;
  cursor: pointer;
}