I recently decided to upskill my CSS knowledge and find out all the new (for me anyway) bits of CSS3. The way I decided to do this was by inspecting some of the styles used in Bootstrap and manually building (no copy-pasting) my own clones of those, researching the new elements when I encountered them. I've learnt loads along the way, and it's been a quite enjoyable journey.
It also means that I will have a nice lightweight CSS style set that looks very Bootstrappy, but isn't.