Custom themes are now back in Gmail
Google has rolled out custom themes in the new UI of Gmail. However, these are not as powerful as the ones they had on the older Gmail interface.
You do not get to pick your own colors and other elements. The options are fairly limited.
What you can do is pick from the light and dark themes and add a background photo. Google is offering a bunch of images. You can also pick from your own Picasa account. You can also upload a new picture or select something from the web.