Creating responsive pages

Hi all. Great theme you guys have here. I’m really liking it.

I want to create some custom pages and layouts for my site. I’m used to using something like bootstrap to achieve the layouts I want. Doing a quick grep on the source files I did not see any bootstrap stuff in the css so it must all be custom css you wrote. Is there anywhere that documents your css? If I want to create a few rows on a page and each row should have 3 responsive columns how might I go about this?

Again thanks for the awesome theme!