As a backend dev, I wish somebody had told me this exact advice earlier on in my career. CSS does have a fair amount of silly legacy shit, but it's really not all that bad once you dive in.
Another good project's source code to learn from is Skeleton. (more simple than bootstrap)
Another good project's source code to learn from is Skeleton. (more simple than bootstrap)
CSS version: https://github.com/dhg/Skeleton
Sass version: https://github.com/WhatsNewSaes/Skeleton-Sass
Edit: For more of a tutorial approach, check out http://learn.shayhowe.com/html-css/