All of Kentico's form controls are wrapped with Bootstrap classes. You'll need to apply a wrapper class to a containing div (maybe content before and after in the form webpart). Then create your styles in your main stylesheet with that wrapper class and the other bootstrap classes. It works pretty slick and easy once you get to figure it out.
If you're not using the webpart and using the widget instead, you can modify the rich text after you enter the form widget on the page OR you can create a container and assign that in the widget.