Source: http://lifehacker.com/enable-googles-hidden-themes-for-the-stock-android-key-1253487329
If you don't love the look of Android's stock keyboard, Google actually has a few other themes built-in. You just need to uncover them with this trick.
You'll need a custom launcher that can access Android Activities for this to work, like Apex or Nova—which we highly recommend downloading anyway
- Long-press on the home screen and head to Shortcuts > Activities from the menu that pops up.
- Choose "Google Keyboard" (or "Android Keyboard (AOSP)" depending on your device and Android version), and head to "Android Keyboard Debug Settings." This will create a shortcut on your home screen.
- Tap that new shortcut and you'll get some advanced settings for your stock keyboard. Choose "Keyboard Theme" to pick a new theme for your keybaord.
You can choose from Basic (which is the pre-Gingerbread theme), Stone, Gingerbread, and IceCreamSandwich (the default on ICS and higher), with a few variations. Pick which one you like, and enjoy!
Google Keyboard Themes | Google Operating System