Customizability of the web is a pet peeve of mine. And I wonder if selecting the color of text and background should really be a function of the website that the text is on.

There is no technical reason why not everybody should be able to set the color for any website any way they want.

I would expect it to be trivial to make a bookmarklet that changes the color of HN. Here is a quick hack that makes HN display green text on black:

javascript:(function(){s=document.createElement('style');s.textContent='*{color: #0a0 !important; background: #000;}';document.head.appendChild(s)})()

Just edit any of your existing bookmarks and put that line into the url field. Now whenever you click that bookmark, the current page turns into green text on black background.