Using Bootstrap classes in Basis

I am told that Basis theme loads Bootstrap 4. However I'm not getting bootstrap classes to display. My objective is to display a teaser as a pre-set Bootstrap card. I used field-groups to create divs with the correct Bootstrap classes, but none appear to load. If there something I need to do first? Have I misunderstood?

I placed the following code in the body of a page to test (after changing it to accept raw HTML). There is no theming at all - not even a button.

<div class="card" style="width: 18rem;">
  <img class="card-img-top" src="..." alt="Card image cap">
  <div class="card-body">
    <h5 class="card-title">Card title</h5>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
    <a href="#" class="btn btn-primary">Go somewhere</a>
  </div>
</div>