There is a single answer to all similar questions, you didn't started math at school from graph theory, isn't it?
You can't be a Web developer without the knowledge of such core technology of the Web as CSS (Others are HTML and JS). And today I will tell you more, not just CSS3, but CSS4 (Future CSS, CSS Next, CSS Modules Level 4). Moreover without CSS, grid layout and CSS architecture you won't be able to understand and to use Bootstrap.
Bootstrap is a framework and you have to "deep learn" first:
To keep in touch with the latest CSS features I recommend http://cssnext.io/ which is a Post-CSS all-in-one plugin covering all the newest features and upcoming ones.