Yes, Microsoft has some nice Windows 7 themes to choose from...but Windows aficionados worldwide have much to offer besides the standard fare. Take, for example, this lovely Animals theme offered by Phrosen on ithinkdifferent.
Unlike many free themes out there, which usually just provide one wallpaper image, Animals offers the full package. The theme includes a wallpaper slide show with ten high-quality photos of dolphins, leopards, polar bears, a cute wolf cub, and other wildlife. It replaces your boring Windows alert tones with animal calls that will really get your attention, and it changes the Windows color palette to a soothing gray. It even switches your screen saver to something appropriate: Window's own Bubbles.
To install the theme, download the zip file and uncompress it, then open the resulting folder and double-click the Animals file. You then customize the theme via the Windows Personalization dialog (right-click your desktop and select Personalize). Depending on your monitor resolution, you may want to adjust the Desktop Background Picture Position; the default is Fit, and I needed to switch to Fill for my laptop.
Animals is a well-thought-out Windows 7 theme that wildlife lovers will appreciate--I highly recommend it.