Looks like they moved to using the SUIT CSS framework - https://github.com/suitcss. Again the framework is being developed by 3 guys working at Twitter.

It may be an extension to the concepts of Bootstrap - but as @mdo has already moved out of Twitter - expecting BS to always be aligned to Twitter would have been more to ask for.

Thus, the concept fork and a new framework.

I think they're in their right to evolve and explore. But I think Bootstrap is not an style, it is just a foundation, so, it doesn't matter what foundation they use, what it matters it is the result of the design, wich, imho, lacks in design taste.

Why did they ditch bootstrap?