Doesn't really describe "how" it happened.
Sure, it's a shame, but css, like html and javascript, is not a protected source. It's run client-side. Would you rather just receive a image/png from websites?
EDIT: I know someone who've experienced something very similar (with css prefixes obviously referring to his site rather than the microsoft.cn clone).
Grid layouts, pinning images, etc. are ideas. Ideas are not protected. The concrete implementations of them are. You can clone Pinterest, but you can't do it by stealing their JavaScript and CSS files. You have to write your own.
Is that a serious question?