body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 0;
    background: #f4f4f4;
}
header, footer {
    background: #222;
    color: white;
    text-align: center;
    padding: 1em;
}
nav a {
    color: #ddd;
    margin: 0 15px;
    text-decoration: none;
}
.app {
    background: white;
    margin: 1em;
    padding: 1em;
    border-radius: 8px;
    max-width: 400px;
}
main {
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
}