I wanted to install the following theme (http://gnome-look.org/content/show.php?action=content&content=155582) and tried to do it like this (How can I install GTK themes on Ubuntu?), however, there's no .deb file in the archive for step 3. How can I install it now?
Open the archive and place its contents in '~/.themes'
'.themes' is located in your home folder. It's a hidden folder that you can make visible by pressing ctrl H.
If you don't have a '.themes' folder; create it.
With this method you are the only user that can use the theme. If you wish to install system-wide use the other answer.
Save the file and open your terminal, run below command:
sudo tar xvzf themeFile.tar.gz -C /usr/share/themes/
Then check your settings or logoff and login to see changes.