header {
    justify-content: center;
}

main {
    display: flex;
}

.game {
    flex: 1;
}

.back {
    width: 30px;
    height: 30px;
    background-color: #000000ad;
    position: absolute;
    right: 0;
    top: 0;
    display: none;
}

@media only screen and (max-device-width: 768px) and (orientation: landscape) {
    header {
        display: none;
    }

    /* 仅限手机，横屏模式 */
    .back {
        display: block;
        z-index: 9999;
    }
}