  <style>
    body {
      margin: 0;
      font-family: sans-serif;
      background-color: #f5f5f5;
    }

    .wrapper {
      max-width: 960px;
      margin: 0 auto;
      background-color: #ffffff;
      min-height: 100vh;
      box-shadow: 0 0 8px rgba(0,0,0,0.1);
      display: flex;
      flex-direction: column;
    }

    header {
      padding: 16px 24px;
      border-bottom: 1px solid #ddd;
      display: flex;
      justify-content: space-between;
      align-items: center;
    }

    .logo {
      font-size: 20px;
      font-weight: bold;
    }

    nav a {
      margin-left: 16px;
      text-decoration: none;
      color: #333;
      font-size: 14px;
    }

    centernav a {
      text-decoration: none;
      color: #333;
      font-size: 25px;
    }

    main {
      flex: 1;
      padding: 24px;
      border-bottom: 1px solid #eee;
    }

    .hero {
      border: 1px dashed #ccc;
      padding: 40px;
      text-align: center;
      color: #777;
    }

    .hero2 {
      border: 1px dashed #ccc;
      padding: 10px;
      text-align: center;
      color: #777;
    }

    footer {
      padding: 12px 24px;
      font-size: 12px;
      color: #777;
      text-align: center;
    }

/* --- ここからスマホ対応（追記） --- */
@media (max-width: 768px) {

  .wrapper {
    max-width: 100%;
    box-shadow: none;
    min-height: auto;
  }

  header {
    flex-direction: column;
    align-items: flex-start;
    padding: 12px 16px;
  }

  nav a {
    margin-left: 0;
    margin-right: 12px;
    font-size: 13px;
  }

  centernav a {
    font-size: 20px;
  }

  main {
    padding: 16px;
  }

  .hero {
    padding: 24px;
    font-size: 14px;
  }

  .hero2 {
    padding: 12px;
    font-size: 13px;
  }

  footer {
    padding: 10px 16px;
    font-size: 11px;
  }
}
/* --- ここまでスマホ対応（追記） --- */
  </style>