e.g. try loading news.ycombinator.com?myColorVar=44ff55 , it will load normally, as usual. Of course, this variable means nothing to the HN backend, but this is where e.g. Grease-monkey comes in: It injects a bit of javascript of your choosing into the page context for the news.ycombinator.com/* pages. This scriplet can easily change the color of the header-bar, and add the colorVariable to all (non-external) links.
Of course, you can add a line like the following to your local userContent.css:
@-moz-document domain(news.ycombinator.com) { #hnmain>tbody>tr:first-of-type>td { background-color: lime !important; }
, but this has the disadvantage of requiring a Firefox restart for each change, whereas my method requires only twiddling the value in the URL, and it's valid for all future requests, until you change it again.
I hope, the above makes it clearer, what I meant the post above