Now you can have different themes for each of your iGoogle tabs. If you select a new theme, you'll notice that only the current tab has a new look. All the other tabs have the old theme. This is also works when you share a tab: the theme is saved and is part of you tab's identity.
Google promised to release an interface that lets you create your own theme, but for the moment a good way to add custom themes is this third-party gadget. Note that you need to add the gadget for each tab you want to have a custom theme and you shouldn't expect to find themes that change depending on weather or time of the day.
{ spotted by Colin Colehour }