要为具有红色背景色的卡设置消极动作（危险），您需要使用Bootstrap上下文类卡危险。
设置危险动作-
<div class="card bg-danger"> <div class="card-body">Danger! High Voltage!</div> </div>
您可以尝试运行以下代码来为Bootstrap 4中的卡设置危险操作-
<!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> </head> <body> <div class="container"> <h2>Messages</h2> <div class="card"> <div class="card-body">How you doing?</div> </div><br> <div class="card bg-danger"> <div class="card-body">Danger! High Voltage!</div> </div><br> </div> </body> </html>