alert-danger

bootstrap5.css

.alert-danger {
  color: #721c24;
  background-color: #f8d7da;
  border-color: #f5c6cb;
}

bootstrap5.css

.alert-danger hr {
  border-top-color: #f1b0b7;
}

bootstrap5.css

.alert-danger .alert-link {
  color: #491217;
}

html bootstrap5 Sample

アラートの背景色：alert-*

alert-primary

alert-secondary

alert-success

alert-danger

alert-warning

alert-info

alert-light

alert-dark

  <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>