Google Personalized Homepage becomes a little bit more "customizable". Now you can select between the default theme and six other childish themes. Themes change the background, text colors, icons, Google's logo, button sizes and more.
The theme is actually an XML that contains a list of CSS files. Google says that the "theme will dynamically change to match your time of day, including local sunrise and sunset times", so you have to enter your location. If you look in the XML, you'll notice a lot of references to the weather, so it's likely that the theme will change depending on weather, as well.
Because the theme is actually just a CSS file (and its dependencies), Google could let users build their own themes and maybe even host those themes to improve the performance.