Gboard gets new Light and Dark gradient themes on Android devices2
Gboard, you might already have access to them. They are not limited to Google Pixel phones, but availability for other handsets seems to be limited to select models.
However, when the update is ready for prime time, all the new themes will be rolled out to everyone, regardless of what phone they use, as long as it's compatible with Google's Gboard app.
According to XDA-Developers, there are 25 Light and 29 Dark gradient themes included in the latest update for Gboard beta. If you've already enrolled in the beta program, you can already try them out by downloading the latest Gboard beta build via Google Play Store.