html, body {
  margin: 0;
  padding: 0;
  height: 100%;
  width: 100%;
  background: radial-gradient(circle, #444, #111);
  display: flex;
  justify-content: center;  /* center horizontally */
  align-items: center;      /* center vertically */
}

canvas {
  display: block;
  /* optional: add a border or shadow if you like */
}