An update is rolling out now for Google Keyboard that offers a pair of Material Design themes for your typing experience. Dubbed "Material Light" and "Material Dark" (with color themes of light gray and dark gray, respectively), the two themes aren't enabled by default on Holo-themed Android 4.4 KitKat and older devices.
By all appearances these new themes match what's available on Android 5.0 Lollipop-powered devices like the Nexus 6 and Nexus 9, as well the Lollipop preview images. They don't really change the behavior of the keyboard, just the appearance, but considering how many apps have received a Material Design update as of late, you might as well make the switch and get used to the new style of things.
The 4.0 update for Google Keyboard is rolling out now on Google Play. Check it out and let us know what you think of the new themes.