要为元素设置浅灰色边框，请使用Bootstrap 4中的border border-light类。
如下图所示，添加边框很容易-
<div class="demo border border-light"> This element has light border </div>
您可以尝试运行以下代码来实现border-light-class-
<!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>Demo</p>
<div class="demo border border-light">This element has light border</div>
</div>
</body>
</html>