The update adds 15 themes in different colors, including black, red, green, blue, and more. You also have the option of adding your own background image as the theme, as well as setting the transparency of the background. Ability to remove key borders is also new in this update.
The update is gradually making its way to users, so if you don't see it right away, hold tight. It's coming. And it'll be great. If you don't have Google Keyboard installed, grab it from the Play Store.