Your question is flawed, and utterly huffing backwards.
In order to have bootcrap do anything it wasn't designed to do out of box, you HAVE to learn CSS; but to be brutally frank (You'll find I'm rarely anything but) you should have a command of CSS LONG BEFORE you even THINK about looking at any fat bloated idiotic halfwit mind-numbingly ignorant asshattery that are HTML/CSS frameworks!
Particularly when how it works is the antithesis of why HTML and CSS are separate in the first bloody place, and it makes you write as much if not more code than you'd have written without it if you had a proper command of HTML and CSS in the first bloody place!
Go find a stick, scrape off the bootcrap, and learn to use HTML and CSS properly BEFORE you dive for the bloated framework chazerei. IF you are able to learn those properly, you will quickly find that NONE of the claims about bootcrap bear any resemblance to reality or sanity. It is not easier, or faster, and it sure as HELL doesn't make you more productive. All you'll end up doing is sleazing out two to six times the markup to write as much custom CSS on top of bootcrap as you'd have without it! Or at least, two to six times the markup if you know what semantics are and how to leverage selectors so you aren't mindlessly idiotically throwing classes at EVERYTHING. As Carlin joked, not every ejaculation deserves a name -- to that end not every damned element needs a div around and three classes atop it.
... and anyone out there making the claims that bootstrap in fact does those things, doesn't know enough about HTML, CSS, progressive enhancement, separation of presentation from content, semantic markup, or accessibility guidelines to flap their yap about it!