body {
    width: 100%;
    overflow: hidden;
    background: url("space.jpg");
    background-size: cover;
}

#board {
    position: relative;
    width: 500px;
    height: 500px;
    margin: 40px auto;
    border: 2px solid white;
    background-color: black;
    overflow: hidden;
}

#jet {
    position: absolute;
    width: 40px;
    height: 40px;
    background: url("rocket.png");
    background-size: contain;
    background-repeat: no-repeat;
    /* jet 在畫面的位置 */
    left: 50%;
    bottom: 0px;
}

.bullets {
    position: absolute;
    bottom: 40px;
    left: 51%;
    width: 20px;
    height: 20px;
    background: url("bullet.png");
    background-size: contain;
    background-repeat: no-repeat;
}

.aliens {
    position: absolute;
    top: 0px;
    width: 50px;
    height: 50px;
    background: url("alien.png");
    background-size: contain;
    background-repeat: no-repeat;
}

#points {
    position: absolute;
    top: 0px;
    margin: 40px;
    opacity: 0.7;
    font-size: 10em;
    color: white;
}