body, html {
    margin: 0;
    padding: 0;
    height: 100%;
}

.hero {
    width: 100%;
    height: calc(var(--vh, 1vh) * 100);
    background-image: url('cs.jpg');
    background-position: center top;
    background-size: cover;
    background-repeat: no-repeat;
}

@media only screen and (max-width: 768px) {
    .hero {
        background-image: url('cs-m.jpg');
    }
}