One such great feature only a few Android users are aware of is – The ability to set background themes and images in Google Keyboard. This allows you to pick from a variety of themes, including different color schemes for the keyboard, or you can even upload one of your favorite pictures as the background image for Google Keyboard.
1. To change the theme, just head into the settings of your Android
phone and then select theLanguage & input option.
2. Now tap on Google Keyboard (Gboard) followed by Theme option.
3. You’ll now find various options to set your keyboard as per your
desired picture or color.
4. You will then be presented with all the folders that contain photos,
select a photo that you like and then adjust it to your preference.
After that, tap on ‘Next‘ and then ‘Done‘.
5. That’s it, your favorite image has now been set as a background
image in Google Keyboard.
Source from: https://www.alltechbuzz.net/change-the-colour-of-keyboard/