nav ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
  }
  
  nav ul li {
    display: inline;
    padding: 10px;
  }
  
  .modal {
    display: none;
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background-color: #f9f9f9;
    border: 1px solid #ccc;
    padding: 20px;
    z-index: 1000;
  }
  
  .modal-content {
    text-align: center;
  }
  
  .close {
    float: right;
    cursor: pointer;
    font-size: 24px;
  }
  
  .close:hover {
    color: #aaa;
  }
  
  .error-message {
    color: red;
  }