I don't endorse a blatant rip off of the UI and the assets. You can do better than that. Otherwise, good job on the development effort and open source initiative.
It is a blatant imitation[1]. At the same time, the design is incredibly refined, so there's only so much room to move away from without making sacrifices solely for the sake of differentiation.
EDIT: [1] According to a comment below, it's a straight-up copy of the CSS. That goes well beyond "blatant imitation."
Which is totally fine for development purposes (prototype) and work in progress mockups. But they should have changed it before putting it on a public-facing .com domain.
I'm going to suggest that it's not even good for prototype work, because it invalidates any claim to clean-room implementation and makes it very easy to cross the line between original work and ripoff.
EDIT: [1] According to a comment below, it's a straight-up copy of the CSS. That goes well beyond "blatant imitation."