Gboard's stock theme is a little boring (and bright), but you can easily change it to something more to your liking. The available options have increased in the latest build of the app, too. There's a new solid color that should match that not pink Pixel 3, as well as a number of slick gradients.
In the latest update, Google has added a selection of dark and light gradient themes. You can see a few samples of them above. There's also the new "sand" solid color. Additionally, Gboard seems to take into account the color of your keyboard at the system level. Dark themes retain the dark navigation bar and light themes get a white bar. This will vary on non-Google devices, of course.
The new themes show up in Gboard v7.8.8 but not for everyone. Some devices we've tested get the new options right away, but there's some server-side component involved here. You can grab the latest version from APK Mirror to see if you're lucky.
- Nick and Diego