要设置蓝色轮廓的Bootstrap 4按钮，请使用.btn-outline-primary类。
将蓝色轮廓添加到按钮-
<button type="button" class="btn btn-outline-primary"> Blue outline </button>
您可以尝试运行以下代码以在Bootstrap中实现btn-outline-primary类-
<!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"> <h4>Bootstrap 4</h4> <p>You can see blue outline below:</p> <button type="button" class="btn btn-outline-primary">Blue outline</button> </div> </body> </html>