When I login into my adesense account, I saw a news:
We’ve added seasonal spirit to our ad units – you now have the option of displaying themed colors and graphics in your ads during holidays and special events. Visit our What’s New page for details.
Adsense is ready for Christmas! Adsense called the new format ads “themed ad units”. They are the ad units that display themed colors and graphics during holidays and special events.
At the Adsense’s Ad Formats page, I only see 160×600 Wide Skyscraper layout is supported. Not sure about others.
What you need to do to activate the themed ad units is go to “My Account”->”Ad Type Preference ” and enable it.