Is it just me? I don't recite the built-in functions (intentionally) before I code. I will look into doc when I want to find a shortcut. It's very efficiently.

Take Bootstrap as an example, I open the following doc: http://v4-alpha.getbootstrap.com/components/alerts/

If I want a Modal, I will check Modal doc (accurately, I jump to CODE PATTERN and copy it, not starting to read doc explanation.) and use it. After few times, you will memorize the basic by heart. (understands the basic, then you can create your snippets or something. Myself, I still open the doc to see the visual examples. Anyway, I coded with a lot of frameworks in different projects at the same time, not just bound to Bootstrap. You need to find your best workflow yourself)

EDITED: "customize things" seems not suitable to the questioner at the moment. If he understands basic CSS, he may not ask this question. :)