Yesterday, Google released Android N Developer Preview 3. One of the inclusions is the new Google Keyboard 5.1.
The main addition here is the presence of themes. You can choose from a bunch of solid color themes for the keyboard. There is also an option to set any image of your choice as a background for the keyboard.
5.1 also brings support for the new emoji that are included in Android N. The previous versions of Google Keyboard could not access the new emoji, meaning you can see them but couldn’t type them, and this update fixes that.
Android Police has an APK for the update, which can be installed on older versions of Android but only the themes will be available as the new emoji aren’t present in the older versions.