Google added theme support for their personalized homepage Google IG to make it look so pretty ! There is currently six new themes beside the default classic Google theme : Beach, Bus Stop, City Scape, Sweet dreams, tea house, and seasonal scape. Marissa Mayer revealed that there are ‘Easter eggs’ hidden in the different themes :

Google’s has programmed its decorations to reflect what’s happening in the outside world. Users are asked to enter their ZIP code so the digital drawings change from day to night and fluctuate with the weather.
The designs also will contain hidden surprises known as “Easter eggs” that will open up with an opportune click at the right time of the day, Mayer said. She wouldn’t reveal any of the surprises, but indicated they will delight high-tech geeks with a sense of humor.

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Many easter eggs have been already discovered like the Pi which appear with the Sweet Dreams theme at 3:14 AM. And by entering your Zip code you are able to see the theme changing according to different times of the day. Unfortunately, or maybe feature will be coming later but, currently you are not able to create skins or use others themes beside the official ones provided by Google.