'Focus Effect' is an amazing Gnome Shell extension that allows to add the blur effect to unfocused running windows. This extension is configured by default to display clutter desaturation and an opacity effect, which can be customized as described below.
You need Gnome Tweak Tool to be able to enable this extension, you can install it with this command from the terminal:
sudo apt-get install gnome-tweak-tool
Here is a video showing how does this extension work:
Sign in to Gnome Shell (version 3.4 is required), then open the terminal and install the focus effect extension with the following commands:
sudo add-apt-repository ppa:upubuntu-com/ext
sudo apt-get update
sudo apt-get install focus-effects
Restart now Gnome Shell using ALT+F2 --> r --> Enter, or logout and log back in. Next, open Gnome Tweak Tool, select the "Shell Extensions" tab, then in the right side of the window turn on the "focus-effects" Extension:
If you want to customize the default effect for unfocused windows (rotation, opacity, etc.), you can simply edit the .ffxrc.json file located in your home folder with this command:
Then enter your own settings:
Credits for this extension go here.