bootstrap4.css
.alert-warning {
  color: #856404;
  background-color: #fff3cd;
  border-color: #ffeeba;
}
bootstrap4.css
.alert-warning hr {
  border-top-color: #ffe8a1;
}
bootstrap4.css
.alert-warning .alert-link {
  color: #533f03;
}
■アラート:alert:コンポーネント:アラートの色

  <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>
■アラート:alert:コンポーネント:アラートを閉じるボタンで、アラート表示を消す

  <div class="alert alert-warning alert-dismissible fade show" role="alert">
    右側の「×」マークをクリックするとアラートが閉じます
    <button type="button" class="close" data-dismiss="alert" aria-label="Close">
      <span aria-hidden="true">×</span>
    </button>
  </div>