向元素添加蓝色背景色以使用Bootstrap设置关键内容
要在元素上设置关键内容，请使用Bootstrap中的.bg-primary类。
您可以尝试运行以下代码来实现.bg-primary类，该类为蓝色-
示例
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Example</title>
<link rel = "stylesheet" href = "https://maxcdn.bootstrapcdn.com/bootstrap/4.1.1/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/4.1.1/js/bootstrap.min.js"></script>
</head>
<body>
<h2>Example</h2>
<div class = "container">
<div class = "w-50 bg-danger">Danger</div>
<div class = "bg-primary">Key text!</div>
</div>
</body>
</html>