style.css

body {
  background-color: #eee;
}

header {
  position: absolute;
  z-index: 2;
  top:50%;
  left: 50%;
  background-color: #fff;
  transform: translate(-50%, -50%);
}

canvas {
  position: absolute;
  z-index: 0;
  width: 100%;
  height: 100%;
  left: 0;
  top: 0;
  right: 0;
  bottom: 0;
  margin: 0;
  touch-action: none;
}