使用border-primary类为元素设置蓝色边框。
对于蓝色边框，添加类，就像为<div>或任何其他元素添加类一样-
<div class="demo border border-primary"> </div>
让我们看一个设置蓝色边框的示例-
<!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: 350px;
height: 170px;
margin: 10px;
}
</style>
</head>
<body>
<div class="container">
<h2>Demo Heading</h2>
<div class="demo border border-primary">This rectangle has blue border</div>
</div>
</body>
</html>