That's what I thought too. Dark means the repo is private. If you fuck up, the world won't see
White means, anything you do here is publicly visible.
Seriously github.com can we please use the black to give some meaning.
I don't want my public repos to have black headers.
At first I assumed this was part of an incremental change to go fully dark, although I'm not sure why they would release it part by part.
But then a tweet [0] seems to imply that it's not gonna happen anytime soon.
[0]: https://twitter.com/mdo/status/830210985512218624