To install Gnome Fallback session DE, you will need to install “gnome-session-fallback” package using Ubuntu Software Center, Synaptic Package Manager (which no longer comes installed by default either in 12.04). Also, Ubuntu Tweak has been discontinued and has been replaced with MyUnity and YPPA Manager, which are very similar functioning applications.
Why switch from Unity Desktop Environment to something else?
To install Classic Ubuntu “Fallback” Desktop Environment:
sudo apt-get install gnome-session-fallback
sudo apt-get install indicator-applet-appmenu
sudo apt-get install gnome-tweak-tool
Then log out and select “GNOME Classic” at the LightDM login screen. IMPORTANT: You need to click on the the little-gear-looking-icon next to where you type your password to change your session to Ubuntu Classic “Fallback” session. And Gnome 3.x Desktop Themes can be added with gnome-tweak-tool.
Hope you find this useful!
Thanks to http://debianhelp.wordpress.com