Themes are files that alter various aspects of GNOME's appearance. Themes can alter the icons that are used throughout GNOME, the appearance of buttons, windows, scrollbars, and so on (collectively called widgets) and can alter the appearance of window wrappings, which consist of the titlebar and border of windows.
To change which theme GNOME uses, one may have to download a theme from the Internet. Gnome Look has a large collection of themes to choose from. If one chooses to browse Gnome Look, the categories to look under are Gtk 2.x, which are widget themes, Metacity, which are window wrapping themes, and Icon themes. One may already have some alternate themes installed on the system.
To change the theme, or install a new one, one must access the Theme Preferences dialog. This can be accessed from the main menu, by going through Main Menu -> System -> Preferences -> Themes.
To install a new theme, one may click on the Install Theme button on this dialog, and find the appropriate theme file. Alternatively one may drag and drop the file from the desktop or file manager, onto the list in the dialog, and this will install the theme.
To change the theme used, one may select from the list visible on the themes dialog. This list contains whole themes, which change the widgets, icons and window wrappings all at the same time. To select widget, icon and window themes individually, one may select a theme from the list, and click on the button labelled Theme Details. A dialog will appear, with tabs respective to changing the widget, icon and window themes that GNOME uses.
Changes made in the themes dialog will be applied instantly, so there is no need to press a button to save the changes.