body {
      font-family: 'Segoe UI', Arial, sans-serif;
      background: #f7f7f7;
      margin: 0;
      padding: 0;
      color: #222;
      /*  ∑, π, ∞, √, ∫ */
      background-image: url('data:image/svg+xml;utf8,<svg width="200" height="200" xmlns="http://www.w3.org/2000/svg"><text x="10" y="40" font-size="32" fill="%23e0e0e0" font-family="Cambria, serif">∑</text><text x="80" y="100" font-size="32" fill="%23e0e0e0" font-family="Cambria, serif">π</text><text x="30" y="160" font-size="32" fill="%23e0e0e0" font-family="Cambria, serif">∞</text><text x="120" y="60" font-size="32" fill="%23e0e0e0" font-family="Cambria, serif">√</text><text x="140" y="180" font-size="32" fill="%23e0e0e0" font-family="Cambria, serif">∫</text></svg>');
      background-repeat: repeat;
    }
    header {
      background: #e8f5e9;
      border-bottom: 2px solid #589358;
      padding: 1.5em 0 1em 0;
      text-align: center;
    }
    nav {
      background: #fff;
      border-bottom: 1px solid #ccc;
      padding: 0.5em 0;
      margin-bottom: 2em;
    }
    nav a {
      color: #589358;
      text-decoration: none;
      margin: 0 1.2em;
      font-weight: bold;
      font-size: 1.1em;
      transition: color 0.2s;
    }
    nav a:hover {
      color: #3333ff;
      text-decoration: underline;
    }
    main {
      max-width: 900px;
      margin: 2em auto;
      background: #fff;
      border-radius: 8px;
      box-shadow: 0 2px 8px #0001;
      padding: 2em 2.5em;
    }
    h1 {
      color: #222;
      font-size: 2em;
      margin-bottom: 0.2em;
    }
    h2 {
      color: #589358;
      margin-top: 2em;
      margin-bottom: 0.5em;
    }
    ul, ol {
      margin-left: 1.5em;
    }
    .section {
      margin-bottom: 2em;
    }
    .important {
      color: #d32f2f;
      font-weight: bold;
    }
    .download-link {
      color: #3333ff;
      font-weight: bold;
    }
    .anki-list a {
      display: block;
      margin-bottom: 0.3em;
    }
    .reserve {
      background: #e3f2fd;
      border-left: 4px solid #1976d2;
      padding: 1em;
      margin-top: 2em;
      border-radius: 4px;
    }
    .math-ref {
      color: #1976d2;
      font-style: italic;
      background: #e3f2fd;
      padding: 0.1em 0.3em;
      border-radius: 3px;
      font-family: 'Cambria Math', 'STIX Math', 'Times New Roman', serif;
    }
    .math-ref a {
      color: #1976d2;
      text-decoration: underline dotted;
    }