The "usability" mistake of HN is the freaking ginormous line-widths. :( I would consider custom stylesheet, but there's no good hook in HN html to grab on to without screwing up other sites. (Seriously, more sites should start adding their domain name as id on html tag. Such as [html id="news-ycombinator-com"].)
A local transparent proxy could add that for you then your custom stylesheet could do what you want. Good OLD Proxomitron comes to mind :) for win boxes.
(I use Safari.)