What does it do exactly?

Displays a Hacker News icon on your browser that changes color when the top story changes.

How does it do it?

The extension opens a websocket connection to the Hacker News Firebase (https://hacker-news.firebaseio.com/) so it can detect new top stories in real time.

Source code at https://github.com/robbiewain/hack-the-news

I really just made it for myself but others have found it useful so I'm posting here. Enjoy!

Are there any more screenshots than just that? What does it do exactly?

An excellent point! I've added a 2nd screenshot and a slightly better description.