Bootstrap is a fat steaming pile of manure, that the ONLY thing you can learn from it is how not to build a website -- by itself uncompressed it is larger than what I'd allow an entire page template of HTML+CSS+SCRIPTS+IMAGES (not counting content) to reach!
It relies on presentational use of classes, usually ends up meaning you write more markup and MORE CSS, does not encourage the use of semantic markup, and on the whole is such a train wreck of ineptitude I am shocked ANYONE would bother trying to use it except out of ignorance on what HTML is, what CSS is, and how to leverage inheritance.
Though as I often say, responsive design is just the next logical step in accessible design after all the things guidelines like the WCAG and the specifications for HTML and CSS have told us for the past decade and a half. IF you've been making semi-fluid elastic layouts with semantic markup, separation of presentation from content, and progressive enhancement, then responsive layout is a no-brainer you should be able to grasp instantly and implement with a dozen or two lines of CSS.
IF however, one has their head still wedged up 1997's backside, using non-semantic presentational markup, stuffing STYLE attributes into the markup, endless pointless classes for nothing, using fixed width layouts with fixed metric fonts, thinking you can actually "design" in a paint program and until recently were still sleazing out HTML 3.2 and slapping 4 tranny on it, today slapping 5 lip-service around that same buggy broken outdated code... well, then everything I'm saying might as well be an alien language; and most likely so pathetically crippled in understanding what a website is -- and is for -- that grasping responsive design concepts is going to be a massive uphill battle.
Which is probably why fat bloated nonsense like LESS, SASS, Bootstrap, jQuery, YUI, Mootools, and all the other idiotic halfwit nonsense that's become industry standard practice have taken off, pissing all over the usefulness, functionality, efficiency, sustainability, maintainability and accessibility of the sites they are used on.
If you have ANY clue what you are doing, you should be able to bring your typical website template in under 8k of markup (not counting content), 32k of CSS (FOR THE ENTIRE SITE), 24k of scripting (FOR THE ENTIRE SITE), and 16k of images (again, not counting content). That's also not counting inept re-re garbage that is forced down our throats by things like social media and advertisers; which by themselves are usually four to five times the size I'd make an entire website.
But really, 72k for your template's HTML+CSS+SCRIPTS+IMAGES, uncompressed, not counting content, social media scripts or advertising scripts. If you can't bring a site template in under those numbers, you probably have NO BUSINESS writing skins for websites...
... and every single framework out there be it HTML, CSS or JS puts you over that number by at least 50% before you even start writing your template.
Which if that's not the pinnacle of developer ineptitude, I don't know what is! Anyone out there sleazing out websites with Bootcrap, do us all a favor and ditch the crutches, take the training wheels off the bike, and put on the big boy pants! Again, I have difficulty fathoming how anyone is DUMB enough to use it by choice, other than a complete ignorance of what HTML is, what CSS is, and how to use them properly.