As a high karma user I get the opportunity to change the color of the top bar. It would be cool if PG allowed me instead to insert some arbitrary CSS then I would make small style changes myself.
I like HN's simple look and feel - my custom CSS would be just a couple of lines to make the damn text bigger on iPhone.
There are helper tools for this, too. Like stylebot for Chrome.
In Firefox the usual way is just to create a userContent.css in your profile folder, then add sections like this:
@-moz-document domain(whatever_domain.com) {
// your site-specific css comes here, use !important for overrides
}