bootstrap4.css
.border-danger {
  border-color: #dc3545 !important;
}
■外枠：Border：ユーティリティ：ボーダー色を設定するクラス
border-primary border-secondary border-success border-danger border-warning border-info border-light border-dark border-white 
<span class="border border-primary">border-primary</span>
<span class="border border-secondary">border-secondary</span>
<span class="border border-success">border-success</span>
<span class="border border-danger">border-danger</span>
<span class="border border-warning">border-warning</span>
<span class="border border-info">border-info</span>
<span class="border border-light">border-light</span>
<span class="border border-dark">border-dark</span>
<span class="border border-white">border-white</span>