Themes are a way to make Sage's web notebook look different. The original theme was developed to resemble Google's gmail, but each user can apply a different theme.
To obtain a theme for your own use only, download it and copy the file to your $HOME/.sage directory.
To make a theme available as the default on a server, download it and copy it to the home directory of the user that is running the server. You will need the appropriate permissions to do this.
To enable the theme, rename it to notebook.css.
Some themes are available here: