Can I just use style.css to impliment new buttons

This is really a basic question so I apologize in advance… I am just getting into Ionic and am liking it so far.

I had some custom buttons that I had created with CSS and js. I would love to be able to implement them. Am I confined to only using app.js, or can I add any js files as long as I import it into index.html? The same with javascript…

I also would love to use some css3 animations. Are those out of bounds?

Any advice would be awesome/ Thank you so much!

Or do all UI Elements HAVE to be the ionic built ones