body {
      font-family: "Roboto", sans-serif;
      margin: 0; padding: 0;
      background-color: #f4f7fc;
      color: #222;
      line-height: 1.5;
    }
    header {
      background: linear-gradient(135deg, #0066cc, #004a99);
      color: white;
      padding: 18px 0;
      text-align: center;
      font-weight: bold;
      font-size: 1.5rem;
    }
    main {
      max-width: 900px;
      margin: 30px auto;
      background: white;
      border-radius: 12px;
      padding: 30px;
      box-shadow: 0 6px 18px rgba(0,0,0,0.1);
    }
    h2 {
      color: #004a99;
      margin-bottom: 20px;
    }
    .exercise {
      margin-bottom: 30px;
      border-bottom: 1px solid #ddd;
      padding-bottom: 20px;
    }
    .question {
      font-weight: 600;
      margin-bottom: 10px;
      font-size: 1.1rem;
    }
    .btn-code-toggle {
  background-color: #0066cc;  /* xanh blue chủ đạo */
  border: none;
  color: white;
  padding: 8px 16px;
  border-radius: 8px;
  cursor: pointer;
  font-weight: 600;
  margin-bottom: 12px;
  transition: background-color 0.3s;
}

.btn-code-toggle:hover {
  background-color: #004a99;  /* xanh blue đậm khi hover */
}
    .code-container {
      display: none;
      border: 1px solid #ccc;
      border-radius: 12px;
      overflow: hidden;
      box-shadow: 0 0 10px rgba(0,0,0,0.1);
    }
    .code-columns {
      display: flex;
      gap: 12px;
      flex-wrap: wrap;
    }
    pre {
      background-color: #f0f0f0;
      padding: 15px;
      margin: 0;
      overflow-x: auto;
      font-family: Consolas, Monaco, "Courier New", monospace;
      font-size: 0.9rem;
      border-radius: 0 0 12px 12px;
      flex: 1 1 45%;
      min-width: 300px;
    }
    .code-header {
      background-color: #004a99;
      color: white;
      padding: 8px 15px;
      font-weight: 700;
      font-size: 1rem;
      border-radius: 12px 12px 0 0;
    }