If you have clib installed do:
clib install puffnfresh/toggle-osx-shadows
Run the following commands in Terminal to toggle OS X window shadows off.
git clone https://github.com/pufuwozu/toggle-osx-shadows.git
cd toggle-osx-shadows
make
./toggle-osx-shadows
brew install https://raw.githubusercontent.com/puffnfresh/toggle-osx-shadows/master/homebrew/toggle-osx-shadows.rb
To turn on shadows again, run ./toggle-osx-shadows
(like in step four above), log out, and log back in.