@media only screen and (max-width: 768px) {
    body {
        background: url('https://api.yimian.xyz/img?type=moe&size=1920-2000x1080-1200');
        background-size: cover;
    }
}

@media only screen and (min-width: 768px) {
    body {
        background: url('https://px3.rainchan.win/img/original/94819771');
        background-size: cover;
    }
}