Stylebot (chrome extension) lets you do that. That's actually what I use for HN, by example. It's a really good extension. Click it, click directly on your pag

Stylebot (chrome extension) lets you do that. That's actually what I use for HN, by example. It's a really good extension. Click it, click directly on your page to select a DOM element, enter any valid CSS, and it's saved for your next visit.