@media only screen and (max-width: 500px) {
    #box-fiche {
        height: auto !important;
    }
    #conteneur-fiche {
        padding-top: 30px !important;
    }
}

@media only screen and (max-width: 900px) and (min-width: 501px) {
    #box-fiche {
        width: 90% !important;
    }
    #bouton-valider {
        width: 80% !important;
    }
}