Mac: Make hidden Dock icons transparent

Open Terminal.app (Applications > Utilities) and type the following command exactly:

defaults write com.apple.Dock showhidden -bool YES

Once you type the command and press enter, you need to restart the Dock by typing "killall Dock" and pressing enter. Once the Dock restarts and you hide applications, you will notice that hidden applications have a transparent look. If you want to reverse the command, just replace the "YES" with a "NO" and restart the Dock again.