I took a few minutes today and setup a style switching mechanism on my site. I didn't do a whole lot to the new stylesheet, just inverted the colors, but I did grab some javascript from A List Apart's article on alternative stylesheets that allows me to switch and save styles. Yes, I could have made something using prototype to switch styles, as I have been intending for sometime now. No, I didn't want to.
I suggest you check out ALA's article, and check it out in action.
Switch to the white stylesheet
Switch to the black stylesheet