要将橙色边框设置为元素，请使用border-warning类-
<div class="mystyle border border-warning"> Danger (Orange border) </div>
设置元素的样式-
<style>
.mystyle {
width: 200px;
height: 100px;
margin: 10px;
}
</style>让我们看一个在Bootstrap中实现border-warning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;
}
</style>
</head>
<body>
<div class="container">
<p>The following are two Rectangles:</p>
<div class="mystyle border border-warning">Danger (Orange border)</div>
</div>
</body>
</html>