body {
    background-color: #000;
    margin: 0;
}

#drawing_canvas {
    position: absolute;
    margin: auto;
    width: 768px;
    height: 768px;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
}

button {
    position: fixed;
    right: 0;
    bottom: 0;
    margin: 8px;
    cursor: pointer;
}