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
<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>