#button {
    position: absolute;
    left:0;
    top: 0;
    width: 100%;
    height: 100%;
    background-color: black;
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    font-size: 500%;
}

canvas {
    position: absolute;
    left:0;
    top: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0,0,0,.2);
}