body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 0;
}

header, nav, main, footer {
    padding: 15px;
    text-align: center;
}

nav ul {
  display: flex;
  justify-content: center; 
  list-style-type: none;
  margin: 0 auto;
  padding: 0;
}

nav li {
  margin: 0 5px;
}

@media (min-width: 768px) {
    main {
        font-size: 20px;
    }

    nav ul {
        display: flex;
        justify-content: center;
    }
}