Themes are typically distributed in compressed tar archives. After you download them to your computer, you can install them by clicking the Install button in GNOME's Appearance Preferences dialog and then selecting the relevant archive in the file chooser. This will generally make the theme accessible through the list in the Appearance Preferences dialog. You can also install a theme from the command line by expanding the compressed archive in the
~/.themes directory. Icon themes are placed in the
You can apply a theme by selecting its thumbnail in the Appearance Preferences dialog. The thumbnails are metathemes, which consist of a combination of other theme elements. You can get more control over your theme settings by clicking the Customize button in the dialog. This will let you choose the widget theme and window border theme individually. In some cases, you can also customize the colors that are used in a theme.
Keep in mind that some of the themes described in this article, such as Sonar and Nimbus, are designed for a specific distro and might not work perfectly in other kinds of environments.
If you are using Ubuntu, be sure to check the package archive before you try to install from a tarball. The Shiki themes and GNOME Colors icons are all available directly through APT. For additional themes, I recommend perusing the GNOME-Look.org website.
via ars technica