html, body {
    margin: 0;
    width: 100%;
    height: 100%;
    display: flex;
    justify-content: center;
    align-items: center;
    background: radial-gradient(circle at center, transparent 0%, #222 0%, transparent 100%), #555555;
}
canvas {
    background: radial-gradient(circle at center, transparent 0%, #222 0%, transparent 100%), black;
}