HN top bar colours by popularity: https://news.ycombinator.com/topcolors
Wait, you can change the top bar color? That's news to me.
At certain karma thresholds, different things unlock. There's a list somewhere but I'm not sure if it's up-to-date.[0] It looks like top bar color can be changed once you hit 250 karma.
List of undocumented HN features: