Ask HN: Why the Black Line on Top?

Rebecca Heineman died: https://news.ycombinator.com/item?id=45960368

Occasionally, there will be a thin black bar at the top of the top bar, in memoriam of a significant figure in the tech/science community dying. A Hacker News submission about the death will usually be on the front page at that time.

https://github.com/minimaxir/hacker-news-undocumented?tab=re...

Yep, symbolic of mourning, acknowledging the passing of a figure of importance or notability to the community.