html, body { height: 100%; }
body {
  background: hsl(256, 18%, 17%);
  background-image:
    radial-gradient(
      circle at top right,
      hsl(256, 18%, 35%),
      hsl(256, 18%, 25%),
      hsl(256, 24%, 16%)
    );
}

canvas {
  position: absolute;
  top: 0; right: 0; bottom: 0; left: 0;
  margin: auto;
  max-width: 100%;
  height: auto;
}