alert-dark

bootstrap4.css

bootstrap4.css
.alert-dark {
  color: #1b1e21;
  background-color: #d6d8d9;
  border-color: #c6c8ca;
}
bootstrap4.css
.alert-dark hr {
  border-top-color: #b9bbbe;
}
bootstrap4.css
.alert-dark .alert-link {
  color: #040505;
}

html Sample

アラートの背景色:alert-*


  <div class="alert alert-primary" role="alert">alert-primary</div>
  <div class="alert alert-secondary" role="alert">alert-secondary</div>
  <div class="alert alert-success" role="alert">alert-success</div>
  <div class="alert alert-danger" role="alert">alert-danger</div>
  <div class="alert alert-warning" role="alert">alert-warning</div>
  <div class="alert alert-info" role="alert">alert-info</div>
  <div class="alert alert-light" role="alert">alert-light</div>
  <div class="alert alert-dark" role="alert">alert-dark</div>