body {
  font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
  margin: 0;
  padding: 0;
  line-height: 1.6;
}

header {
  background: #333;
  color: #fff;
  padding: 1rem;
}

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

header nav ul li {
  margin: 0 1rem;
}

header nav ul li a {
  color: #fff;
  text-decoration: none;
}

footer {
  text-align: center;
  padding: 1rem;
  background: #f4f4f4;
}

main {
  padding: 2rem;
}

@media (max-width: 768px) {
  header nav ul {
    flex-direction: column;
    align-items: center;
  }

  header nav ul li {
    margin: 0.5rem 0;
  }
}