html, body {
  margin: 0;
  padding: 0;
  height: 100%;
  display: flex;
  justify-content: center;
  align-items: center;
  background: radial-gradient(circle at center, #222, #000);
}

canvas { display: block; border: 2px solid rgba(255,255,255,0.15); box-shadow: 0 10px 40px rgba(0,0,0,0.6); border-radius: 8px; }