How To Install New Themes In Ubuntu 15.04 14.10 14.04 13.04 12.04

1)
Download Theme
Goto http://www.ubuntuthemes.org
Download a theme
For example http://www.ubuntuthemes.org/wood.html
http://www.ubuntuthemes.org/wp-content/uploads/2011/07/Wood.tar.gz

2)
Install ubuntu-tweak
#sudo add-apt-repository ppa:tualatrix/ppa
 
#sudo apt-get update -o Dir::Etc::sourcelist="sources.list.d/tualatrix-ppa-saucy.list" -o Dir::Etc::sourceparts="-" -o APT::Get::List-Cleanup="0"
#OR
#sudo apt-get update

#sudo apt-get install ubuntu-tweak

3)
OR 
Install Unity-Tweak-Tool
#sudo apt-get install unity-tweak-tool

4)
Extract downloaded file to ~/.themes folder

5)
Goto "Tweaks" and click "theme" and select your theme