bootstrap4.css
.alert-danger {
color: #721c24;
background-color: #f8d7da;
border-color: #f5c6cb;
}
bootstrap4.css
.alert-danger hr {
border-top-color: #f1b0b7;
}
bootstrap4.css
.alert-danger .alert-link {
color: #491217;
}
■アラート：alert：コンポーネント：アラートの色
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>