Actually Buddy
there is a lot of difference between designing and developing.
Designing means to meet the requirements .
Developing is to code that design which had meet your requirement .
Bootstrap is actually a framework made entirely on CSS by two dudes in twitter .They made it for many reasons the major one is to acquire responsive view.
CSS by learning it you will acquire the art of making a webpage awesome while Bootstrap will make it responsive against all screens .
So YES learn the CSS .
Cheers ...