UPDATE: There is an easier way to install Gnome Shell themes. Use this method.
To install the theme, first backup the original Gnome Shell theme (This is necessary to be be able to go back to the default Gnome Shell theme). Then, extract the downloaded theme zip file and rename “gnome-shell” folder to “theme”. Copy the theme folder to /usr/share/gnome-shell/ merging and replacing the original theme folder.
Restart Gnome Shell by pressing Alt + F2 and then running r.
More detailed instructions, including a video, on installing Gnome Shell themes can be found at my post here.
Images courtesy of half-left.