How do I install Application themes?
Installing an Application theme can be done in two ways. You must download the theme package first. Then, from the panel menu, navigate to Desktop Preferences->Theme. Click on the "Theme Details" button, then click on the "Install Theme". Select the theme package you downloaded using the "Browse" button. Then click okay. Some themes may offer a Window border theme included inside the Application theme. That just saves you one extra step.
The other way of installing a theme is to manually do it. Open a terminal (Panel menu->System Tools->Terminal), change directory into ~/.themes and use tar to unpack the theme. For example, a theme called America, you would type: cd ~/.themes && tar xzf /America.tar.gz. Be sure to replace the "/America.tar.gz" with the actual path of the file you downloaded.
There is one final way of installing themes, which is similar to the terminal method. You may use the Archive Manager to open the theme package by double-clicking on it. Then use the "Extract" button, and extract it to your theme folder (accessible by clicking "Go to Theme Folder" in the Theme Details window.
the above was taken from:
http://art.gnome.org/faq.php#q2
sometimes gnome throws a hissy fit complaining that it is not a valid theme, so you have to manaualy extract the contents and just put them in your ./themes folder in your home directory e.g. /home/Michael_aust/.themes
