Google’s new layout seems to have made it easy for them to add themes to their services.
Gmail of course has supported themes for a long time now and Google updated the themes on the new layout. iGoogle has also supported themes for a long time now.
Google Groups is another service that has received theme support now. One theme is available right now. The default one features a white colored background. The optional theme features a soft gray color.
Remains to be seen if we would see theme support appearing in other Google services in the future!