It's MIT-licensed open source.
I've been using a fork (also MIT): https://github.com/alexferrari88/refined-hacker-news
One thing I miss is the orange mark identifying the OP of a post.
https://github.com/OrangeJuiceExtension/OrangeJuice/blob/b84...
There was no [OP] label when I first reloaded this page, and now after replying, I am marked as the [OP].
edit: it seems the [OP] relies on a URL hash with the ID of the OP. However, this doesn't work for me because I don't navigate to HN posts from the HN site.
I usually come from https://hn.leftium.com. (Or a page like https://hw.leftium.com/#/item/47694036)
https://github.com/plibither8/refined-hacker-news/blob/ee7ef...