Cause astigmatism: http://blog.tatham.oddie.com.au/2008/10/13/why-light-text-on...

I absolutely hate reading websites that do light on dark.

I use this Chrome extension and it works great. http://www.chromeextensions.org/appearance-functioning/chang...

I used to use some extension on firefox as well, can't recall the name.

EDIT: Perhaps this https://addons.mozilla.org/en-us/firefox/addon/color-that-si...