We got to know about it through the first Android Developer Preview and now it has finally gone official. We are talking about the Google Keyboard v5.1, which is rolling out officially via the Google Play store.
This is just the Google Keyboard and not the Google Indic Keyboard that already seems to support the new themes and customisations (right from the Play Store). The current version of the Google Keyboard v5.0 only supports the outline for keys and two themes. The new update changes all of this.
The new v5.1 update adds plenty of new themes and these can be accessed via the theme selector. If you are not happy with those Material Design themes, you can even add your favourite image as the background and tap away.
In the new theme menu, there is once again the key border toggle and also support for Unicode 9 that uses selectable skin tones. As reported earlier, the same will not be available until you upgrade to Android Nougat.
The update is rolling out gradually, but you can download and sideload the .apk file if you are in a hurry. Also keep in mind that you download the right variant that made for your device type.