I rarely use any of the popular CSS frameworks. Unless you are making a small(ish) website where the styling is not important, it is actually more work to customize an existing framework than to just use CSS yourself.

But what I do use sometimes is a CSS reset like https://github.com/necolas/normalize.css

And what I also use is this CSS boilerplate: https://github.com/frontaid/natural-selection (Disclaimer: I created it because I wanted to avoid the repetitive work of writing out all the selectors).