It’s called “Ambiru,” by Phu Ly, and it’s described as “calm” and “reassuring.” Not inconsiderable virtues, these.
I did, however, look through the entire WordPress.com theme viewer, all 77 themes they claim only “over 60” in order of popularity, to see how Ambiru ranked against the big boys like Chaotic Soul and Supposedly Clean. It never came up. Presumably it was excluded because it was already in use. I am not, however, inclined to install another one in its place just to see where Ambiru really ranks, especially if I have to make up another custom header.