使用border-primary类向Bootstrap 4中的元素添加蓝色边框，如下所示：
将类设置为-
<div class="demo border border-primary"> Blue Border </div>
您可以尝试运行以下代码以实现border-primary类-
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://cdn.staticfile.org/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<style>
.demo {
width: 250px;
height: 170px;
margin: 10px;
}
</style>
</head>
<body>
<div class="container">
<h3>Demo Heading</h3>
<div class="demo border border-primary">Blue Border</div>
</div>
</body>
</html>