To quote fellow News Writer and Member Advocate Admin egzthunder1:
Theming has been as popular on XDA as ROM cooking and development in general. The ability to customize the appearance of our devices is always an appealing concept for many of our members.
Theming can be a lot of fun because it helps personalize a device in ways beyond simply changing a wallpaper. You can theme virtually anything on an Android phone. Now, you can learn how to theme your own keyboard if you own one of the US variants of the Samsung Galaxy S III.
XDA Senior Member rompnit has released a manual method to theme your own keyboard. Finding a theme for your keyboard can be tricky because there are only a finite number that themers release. Now those who what something truly their own can simply make their own. The process is a little complicated and requires—more or less—the same steps involved in theming any APK.
To get started, you’ll need APKTool as well as your favorite zip program, like 7zip. From there, you tear down the keyboard APK you wish to theme, theme it, recompile it, and get it on your phone. It sounds easier than it is, as the process will take some time and effort. However, rompnit does an excellent job of keeping the explanations as simple as possible so even beginners can get in on the action.