Personally, I prefer reading light text on dark background. Most web-pages use black text on white background, including this blog. I'll probably change the theme on this blog later, but I'm actually lazy because of this amazing tool. All you need to do is drag it on your bookmarks toolbar, and voila! One click, and all the text in the page will be grey on black, and links blue on black.
This only works on Firefox. Internet Explorer users; now you have another reason to switch ;-)
Here is the article and the bookmarklet.