Availability: •Consumer: Optional Gmail theme •Google Apps Rapid Release: Optional Gmail theme •Google Apps Scheduled Release: Optional Gmail theme How to: 1. In Gmail, click the gears icon (), and then choose Mail settings. 2. On the Settings page, click the the Themes tab. 3. Click either the Preview or Preview (Dense) theme. You can find more info on their gmailblog post titled "A preview of Gmail’s new look" The new Gmail interface will eventually expand dynamically to accommodate different screen sizes and user preferences, but until then, you can pick the information density that you prefer.
I think email themes will be compatible to the new gmail looks. It shouldn't create error since its google's features.
I love the new design though I also hope that it won't generate much error in loading the page than the previous gmail feature