Google recently sent a note to all their publishers about changing the default google theme. The default theme used was 'Mother Earth' and there is a border.

We're writing to let you know about a coming change to the appearance of your Google ads. Your ads currently display the default Google color palette, Seaside (formerly known as Mother Earth). In the near future, we plan to update the default palette to Open Air, a new palette containing the same set of colors, but without the blue border. We've found that many publishers prefer the cleaner look of this palette and have also seen that a blended color palette performs better for them - attracting user interest while still maintaining the distinction between ads and content with the 'Ads by Google' label.

As most serious adsense publishers know by now, having borderless ads that blend well with your content is the best way to go. However, many adsense publishers specially the novice ones does not even change the default theme.

This move by google would most likely bring in more income to a lot of small publishers as well as increase google's overall advertising income as well.