What I do is take CSS code I might use and put it in a separate text file. Then if I'm editing an ePub, I can copy the already made code.
The best thing to do is create a reset.css and use that as the template. Then copy from it as needed to fix up the ePub. Then you'll make sure you have the correct classes. if your reset.css has a class called noindent and the ePub you are working on has the same class called nonindent, you can easily fix things. But if you were to replace the existing CSS with your reset CSS, every instance of nonindent would fail to render properly.