html {
    height: 100%;
}

body {
    display: flex;
    height: 100%;
    background-color: #333;
}
header {
    display: flex; /* Ajout */
    align-items: center; /* Ajout */
    width: 100%;
    height: 100%;
}
.word {
    color: white;
    font-size: 2.5em;
    font-weight: 800;
    text-align: center;
    top: 50%;
    margin: 0 auto;
    transform: translateY(-50%);
    padding: 0 250px;
}

@media screen and (max-width: 900px) {
    .word {
        font-size: 1.5em;
        padding: 0 50px;
    }
    
}