OnePlus
Change Keyboard Theme (Gboard) auf OnePlus
Gboard on OnePlus ships with a plain gray layout by default. That flat look has nothing to do with your wallpaper or home screen colors, since the keyboard theme is a separate setting inside the keyboard app itself, not part of the general system theme.
To change it, open Settings ➔ System ➔ Languages & input ➔ Virtual keyboard ➔ Gboard ➔ Theme.
From the theme list, pick a design that fits your wallpaper or preferred color scheme. Below the theme selection you can also enable or disable Key borders, which adds a visible outline around each key. This helps if you often mistype on the default borderless layout.
Confirm your choice by tapping Apply. The new theme applies instantly across every app that uses Gboard, including messaging and browser apps.
If the theme list looks empty or only shows one option, Gboard may need an update from the Play Store first. A missing Theme entry usually points to an outdated keyboard app rather than a hidden setting.