So my biggest immediate complaint is that the up/down hearts are misleading. I was a little excited that the uphearts/downhearts appeared to be slightly offset, which would deal with one of HN's most annoying design flaws -- accidentally hitting the wrong vote button. As it is, your hearts have the same horizontal hitbox even though they appear visually skewed. Having the hitbox match the appearance of the hearts would be welcome!

The other fix I would make is in the font-color of a few elements that are now more emphasized than they need to be. e.g. The "unvote" label seems to be in the darkest shade of font color.

As someone who doesn't mind that it is a Chrome plugin, since Chrome is the browser I use for HN and because I don't mind that the plugin has access to that page -- I say keep it as a plugin rather than make it a CSS sheet. With a plugin, you could provide a polished config panel experience that makes it easy for a user to disable/enable features. For example, I could see myself liking the "Hacker News" masthed back in the header and in a hot pink shade -- but other users may want it gone like it is now.

Of course as a plugin it could add all sorts of features to the HN page, at risk of breaking the minimalist experience. All I think I would want in extra features is maybe he ability to switch between a few different variants of pink styling.