要将白色边框设置为元素，请使用border-white类。
添加白色边框-
<div class="mystyle border border-white"> This has white border </div>
上面，我还添加了一种样式来设置div元素-
.mystyle {
width: 200px;
height: 100px;
margin: 10px;
background: blue;
}让我们看一下在Bootstrap中实现border-white类的示例-
<!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>
.mystyle {
width: 200px;
height: 100px;
margin: 10px;
background: blue;
}
</style>
</head>
<body>
<div class="container">
<p>The following are two Rectangles:</p>
<div class="mystyle border border-white">This has white border</div>
<div class="mystyle border border-success">This has orange border</div>
</div>
</body>
</html>