<div class="box-message bg-yellow exclamation-triangle" role="alert"> <p>This is an alert with a warning icon.</p> </div>
.box-message is the main class for building the alert (padding, font size, ...).
.bg-yellow allows you to change the background-color and the color, you can find the list of availables colors here.
.exclamation-triangle allows you to change the icon, you can find the list of icons here.
Thanks to these 3 types of class, you can have a multitude of combinations to display your messages/alerts.