body {
    margin: 0;
  }
  
  ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
    width: 25%;
    background-color: #f1f1f1;
    position: fixed;
    height: 100%;
    overflow: auto;
  }
  
  li a {
    display: block;
    color: #000;
    padding: 8px 16px;
    text-decoration: none;
  }
  
  li a.active {
    background-color: blue;
    color: white;
  }
  
  li a:hover:not(.active) {
    background-color: #555;
    color: white;
  }
  .dropdown {
    position: relative;
    display: inline-block;
  }
  
  .dropdown-content {
    display: none;
    position: absolute;
    background-color: #f9f9f9;
    min-width: 160px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
    z-index: 1;
  }
  
  .dropdown:hover .dropdown-content {
    display: block;
  }
  
  .desc {
    padding: 15px;
    text-align: center;
  }
  a[target=_blank] {
    background-color: yellow;
  }