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-
<!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>