While iGoogle already offers 11 themes to customize the homepage, there's no option to create your own theme. Custom iGoogle Skins is an inline gadget that lets you choose between more than 70 user-created themes (some of them are illustrated in this gallery) or create a custom theme.
An interesting new feature from the latest version of the gadget adds rules for displaying themes: you can choose to show a theme only when it rains, on a certain day of the week or if your IP matches a certain value. Google's themes are already rule-based, but you can't switch between them automatically. For example, this gadget lets you have different themes for a tab if you load iGoogle at home or at work (assuming the IP addresses are static).
Some interesting personalization rules for the feature could include changing the theme depending on your mood, your Google Talk status, your Google Calendar agenda the tab's title or the latest news.