使用Bootstrap 4中的border-light类，为元素设置浅灰色边框，如下所示
<div class="demo border border-light"> light border </div>
演示类的样式用于元素-
.demo {
width: 150px;
height: 220px;
margin: 15px;
}了解如何在Bootstrap中使用border-light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: 150px;
height: 220px;
margin: 15px;
}
</style>
</head>
<body>
<div class="container">
<p>Bootstrap 4</p>
<div class="demo border border-light">light border</div>
</div>
</body>
</html>