body {
    height:100vh;
    width:100vw;
    background-color: #888888;
    margin:0;
    padding:0;
    text-align:center;
}

canvas {
    border: 5px inset #aaaaaa;
    border-radius:10px;
    padding: 0;
    margin: auto;
    display: block;
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
}