body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 0;
    display: flex;
    height: 100vh;
  }
  
  .sidebar {
    width: 250px;
    background-color: #f4f4f4;
    height: 100vh;
    overflow-y: auto;
    padding: 20px;
    box-shadow: 2px 0 5px rgba(0,0,0,0.1);
    flex-shrink: 0;
  }
  
  .main-content {
    flex-grow: 1;
    padding: 20px;
    overflow-y: auto;
  }
  
  .sidebar ul {
    list-style: none;
    padding: 0;
    margin: 0;
  }
  
  .sidebar li {
    margin-bottom: 8px;
  }
  
  /* Parent link styling */
  .sidebar a[data-expand] {
    display: flex;
    align-items: center;
    text-decoration: none;
    color: #333;
    user-select: none;
  }
  .sidebar a[data-expand]:hover {
    color: #007BFF;
  }
  
  /* Arrow */
  .expand-btn {
    margin-right: 6px;
    font-weight: bold;
  }
  
  /* Sub‑topics */
  .subtopics {
    list-style: none;
    padding-left: 1.5em;
    margin-top: 4px;
  }
  
  .subtopics li {
    margin-bottom: 6px;
  }
  
  .subtopics a {
    text-decoration: none;
    color: #555;
  }
  .subtopics a:hover {
    color: #007BFF;
  }