Why isn't my Bootstrap background color class working as expected?

Question

How can I change the color of the modal header? I tried this, but it doesn't work. Did I do something wrong?

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" integrity="sha384-T3c6CoIi6uLrA9TneNEoa7RxnatzjcDSCmG1MXxSR1GAsXEV/Dwwykc2MPK8M2HN" crossorigin="anonymous">

<div class="modal d-block" tabindex="-1">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header bg-body-tertiary">
        <h1 class="modal-title fs-5 " id="exampleModalLabel">Oznámení</h1>
        <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
      </div>

      <div class="modal-body bg-warning">
        <p>Modal body text goes here.</p>
      </div>

      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div>
  </div>
</div>