■色Color:bg-primary:ユーティリティ:背景に色を付ける

<div class="text-center">
  <p><a href="#" class="bg-primary text-white d-block py-2">bg-primary</a></p>
  <p><a href="#" class="bg-secondary text-white d-block py-2">bg-secondary</a></p>
  <p><a href="#" class="bg-success text-white d-block py-2">bg-success</a></p>
  <p><a href="#" class="bg-danger text-white d-block py-2">bg-danger</a></p>
  <p><a href="#" class="bg-warning text-dark d-block py-2">bg-warning</a></p>
  <p><a href="#" class="bg-info text-white d-block py-2">bg-info</a></p>
  <p><a href="#" class="bg-light text-dark d-block py-2">bg-light</a></p>
  <p><a href="#" class="bg-dark text-white d-block py-2">bg-dark</a></p>
  <p><a href="#" class="bg-white text-dark d-block py-2">bg-white</a></p>
  <p><a href="#" class="bg-transparent d-block py-2">bg-transparent</a></p>
</div>