There is already a keyboard theme function in-built for Samsung keyboard, which was introduced in Android 10. But the default theme options are still limited (Android 11).
You can either install themes from Play Store or Galaxy Store, or you can use "Theme Park" or "Keys Cafe" Good Lock plugins to make your own keyboard themes. After you create a theme from those plugins you need to install them, and those themes will show up in Samsung Keyboard > Three dots > Theme. You can change your keyboard theme from there.
Note that the keyboard themes will not work in Dark Mode. You have to disable it to see the applied keyboard theme.
