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sudo add-apt-repository ppa:erik-b-andersen/rgba-gtk

sudo apt-get update && sudo apt-get upgrade

sudo apt-get install gnome-color-chooser gtk2-module-rgba

sudo apt-get install murrine-themes

System > Preferences > Gnome Color Chooser and on the "Engines" tab

check the "Global" box and then select "Murrine" from the drop-down 

click the "Preferences" button next to it and scroll down to 
"Configuration of Enable/Disable RGBA support" and check both of the 
boxes next to "Enable/Disable RGBA support"

Now log out and log back in (required!)