Original post by: Adam Hintz ,
Similar to what machead3 said, it could be Universal Access. To test this on your Mac, simply press ctrl-opt-cmd-8 to turn it on and off. It should be disabled by default. This reverts colors, which makes it easier to see text (white on black is easier on the eyes than black on white). I use this sometimes on my MacBook when viewing a white page and I need to save battery - I can have the contrast almost all the way down and the colors inverted and still see and read text fine.