I've read some guys have done a trick editing this file, but I'm not a guru.
defaults write com.apple.CoreGraphics \
(DisplayUseInvertedPolarity -bool YES)
Basically I want to change the white background color to sepia or anything darker when I read in Skim pdf reader. Since Skim (unlike Kindle) doesnt support this feature, maybe I find a work around and I can change the white color of the screen so it will be useful with any application I want if I can make this happen.