#qr-canvas {
  margin: auto;
  width: calc(100% - 20px);
  max-width: 400px;
}

#btn-scan-qr {
  cursor: pointer;
}

#btn-scan-qr img {
  height: 10em;
  background: white;
  display: block;
    margin-left: auto;
    margin-right: auto;;
}