body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

.nav-container {
    background: white;
    padding: 40px;
    border-radius: 20px;
    box-shadow: 0 20px 60px rgba(0,0,0,0.3);
    text-align: center;
}

.nav-container h1 {
    margin-bottom: 30px;
    color: #667eea;
}

.button {
    display: inline-block;
    padding: 15px 30px;
    margin: 10px;
    border-radius: 12px;
    font-size: 1.1em;
    color: white;
    text-decoration: none;
    transition: transform 0.2s, box-shadow 0.2s;
}

.button-lesson {
    background: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
}

.button-exercise {
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
}

.button:hover {
    transform: translateY(-3px);
    box-shadow: 0 10px 25px rgba(0,0,0,0.2);
}

/* Responsive */
@media (max-width: 768px) {
    .nav-container {
        padding: 25px;
    }
    .button {
        padding: 12px 25px;
        font-size: 1em;
    }
}

/* Correction boutons mobiles */
@media (max-width: 480px) {
    .button {
        display: block;       /* forcer un bloc */
        width: 100%;          /* prend toute la largeur du container */
        max-width: 100%;      /* ne dépasse jamais */
        box-sizing: border-box; /* inclut padding dans la largeur */
        margin: 10px 0;       /* espacement vertical */
    }
}