In order to assign custom styling to a Mendix widget, you may want to assign a class to it and then use this class in your custom sass file. Here the documentation of how to style your app using sass and Calypso:
https://docs.mendix.com/howto/front-end/calypso
And here the learning path:
https://academy.mendix.com/link/path/105/Style-your-App-with-CSS