Though it does not allow you to pick your own colors, the update is certainly good, as it adds 15 color themes, as opposed to the measly 2 it offered prior to the new version’s release.
Google Keyboard with themes, originally debuted for Android 7.0 Nougat, but now works on all devices. You can find the option to switch themes, directly in the main screen of the app’s settings, (earlier it was under Settings > preferences).
New Settings UI
Old Settings UI
The more interesting option, to add your own custom background to the keyboard can also be found under the Theme option. Hit the image selector on the top of the themes screen, and select an app to pick your image.
You will then be able to zoom in/out to scale the image, adjust the brightness and et voila, your new theme is ready.
In addition to themes, the Google Keyboard 5.1 update adds support for gesture typing and word predictions in the following languages: Albanian, Arabic, Basque, Bosnian, English (Australia), English (India), Esperanto, Irish, Javanese, Mongolian, Montenegrin (Cyrillic, Latin), Nepali, Portuguese (Africa), Sundanese, Tajik, Tatar, Zulu.
There is now a proper layout support for Burmese, Southern Sotho, Turkmen, Urdu, Welsh, and Xhosa. The Suggestion Bar now also contains your recently used emojis. Other miscellaneous patches include bug fixes for Talkback input for passwords and missing key labels.
The Google Keyboard 5.1 Update is already available at the Play Store, so hit the link above, or manually check for an update using your device.