This command can be performd in Mac OS X Snow Leopard (10.6) and Lion (10.7) to get folders in Quick Look preview to show up with transparent covers. This option gives you an “x-ray” style view of the icons inside a folder.
defaults write com.apple.finder QLEnableXRayFolders -bool true
To make the command take effect, you will need to restart the Finder
Revert to the default setting:
defaults delete com.apple.finder