To set dark grey outlined border to a button, use the btn-outline-dark class.
Include the brn-outline-dark class as if you add any other class to an element. Here, we have the button element since we want a dark grey outline for our button:
<button type="button" class="btn btn-outline-dark"> Dark gray outline </button>
Let us see an eample to implement the btn-outline-dark class in Bootstrap −
<!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://ajax.googleapis.com/ajax/libs/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>Learning btn-outline-dark class usage:</p> <button type="button" class="btn btn-outline-dark">Dark gray outline</button> </div> </body> </html>