html, body {
    height: 100%;
    margin: 0;
    display: flex;
    flex-direction: column;
}

header {
    background: #333;
    color: #fff;
    text-align: center;
    font-size: 24px;
    font-weight: bold;
    padding: 15px 0;
}

#map {
    flex: 1;
    width: 100%;
}