/* 全体の高さを確保 */
  html, body {
    height: 100%;
    margin: 0;
  }

  body {
    font-family: Arial, sans-serif;
    line-height: 1.6;
    margin: 0;
    padding: 0;
  }
  
  header {
    background: #333;
    color: #fff;
    padding: 1rem 0;
    text-align: center;
  }

  .header-content h1 {
    margin: 0;
    font-size: 2rem;
  }
  
  nav ul {
    list-style: none;
    padding: 0;
  }
  
  nav ul li {
    display: inline;
    margin: 0 15px;
  }
  
  nav ul li a {
    color: #fff;
    text-decoration: none;
    font-weight: bold;
  }
  
  main {
    flex: 1;
    padding: 20px;
  }
  
  footer {
    background: #333;
    color: #fff;
    text-align: center;
    padding: 1rem 0;
  }