Kali Linux is the successor of Backtrack. It follows Backtrack 5rc3 as the next version. Backtrack was based on Ubuntu, but Kali linux is based on Debian. And further more the default desktop that is gnome has been hacked and modified to a large extent. It is gnome 3.4 but is made to appear like a gnome 2 desktop. So basically its a hacked desktop.
By default Kali Linux gnome desktop, does not have any utility for changing or modifying the themes or fonts. So a utility called gnome tweak tool has to be installed first.
$ apt-get install gnome-tweak-tool
Now go to Applications > System Tools > Preferences > Advanced Settings. It will popup the Advanced Settings dialog. Change the theme from the themes tab.
To install a new theme, download the theme and extract the files in the following directory