The only thing you'll need to have in order to access the themes is a custom launcher. Our favorite is Nova Launcher, which is free, so that will do just fine (the download link will be below). Once you're in the custom launcher, just long-press on the homescreen and choose Shortcuts, then Activities. In the menu that pops up, choose "Google Keyboard" (it may be listed as "Android Keyboard (AOSP)"), and then choose "Android Keyboard Debug Settings." Once you've done that, there will be a new shortcut created on your homescreen that will launch you straight into the advanced settings for the stock keyboard, where you will be able to choose your theme.
There are 4 basic options with 6 total options. There is the Basic (which is the pre-Gingerbread theme), Stone, Gingerbread, and Ice Cream Sandwich (the default on ICS and higher), then with Basic there is normal and high contrast, and Stone has normal and bold. Have fun with it!