I believe this is the stylesheet: https://github.com/Primigenus/Cleaner-Hacker-News.

i've been trying out some of the hacker news style extensions, and this one suffers from the same problem as the others -- when you're reading comments, and the comments are centered without a background color for the content div, you can't figure out how deeply nested comments are. it makes it difficult to figure out which comments are top level and which are responses.

That's a good point that's worth addressing. What do you suggest? Left-aligning the content to the window border or something else? Pull requests to the repo on github welcome!

I would actually love a line à-la reddit that shows the nesting level clearly but subtly.

Yes, that's the one. I made a couple changes to mine, but it's essentially the same.