.hero-img {
    position: fixed; /* reste immobile au scroll */
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;

    z-index: -1; /* derrière le contenu */
}

.hero-img{
    background: 
        linear-gradient(to top, rgba(255, 255, 255, 1), rgba(255, 255, 255, 0)),
        url('../img/hero.jpg') no-repeat center center;
    background-size: cover;
    background-position: 30% center;
}

body.dark .hero-img{
    background: 
        linear-gradient(to top, rgba(0, 0, 0, 1), rgba(0, 0, 0, 0)),
        url('../img/hero.jpg') no-repeat center center;
    background-size: cover;
    background-position: 30% center;
}

h1 {
    padding-top: 10rem; /* décalage vers le bas */
}