body {
    width: 100%;
    height: 100vh;
    padding: 0;
    margin: 0;
    box-sizing: border-box;
    background: #333;
    display: grid;
    overflow: hidden;
}

#game {
    margin: auto;
    padding: 0;
    background: #eee;
    border: 3px solid #000;
}

#fullscreen {
    position: fixed;
    right: 10px;
    top: 10px;
    background: transparent;
    color: white;
    padding: 10px;
    border: 1px solid white;
}