Reply To: Modify the theme after I've added this code

[JeroenR]

It is impossible to tell you what you can do without seeing any HTML. So without something like an example in codepen we can’t really help you.
My best guess is that the class btn-default (which is quite a common name) is used in several buttons. So when changing that, you will affect every button that uses this class.
So you will have to find the selector that makes your add to cart button unique. And override that styling.
That selector can either be a style on that button or somewhere in the context of that button.