Thanks for this.
I love the concept, and would use it daily if there were features for adding/removing/viewing favorites. This is something that the official API doesn't support, so it's not something I'm faulting this application for. Impressive for sure.
Also +1 on wlqlwlql's dark-mode idea, although a color theme in general might be a better approach (but perhaps overly complicated?).
The auth string of a story for "fave" is the same as the auth string for the "hide" and "flag" links (since they are already present, the links' hrefs contain that auth string), and so we can extract the auth string from their hrefs and use it for favoriting and un-favoriting.
Moreover, I've created a simple API which can be used to get all the favorited items of a user: https://github.com/plibither8/hn-faves-api