That tutorial is for installing themes from one specific designer. Check out the site www.gnome-look.com for all sorts of other themes. Use the GTK2 filter to find ones better suited to Ubuntu's default Desktop Environment.
Most themes come in a .tar.gz format. To install them simply open up the Appearences app. (System > Preferences > Appearance) and click the themes tab. Once there, click on install new theme and a file browser should open. Navigate to where you saved the (theme).tar.gz file and select it. Ubuntu will then install the theme for you. If the install instructions happen to be different, the author usually posts them on the theme download page.
If you're looking for some cool themes, check out Elementary, Victory, and the
Faenza Icon Set. They are my favorites!