Most Windows 7 installs come with additional themes that can not be selected. But they're there on your computer if you want to check them out. To see the hidden themes open a Windows Explorer window and enter this directory path in the address bar: C:\Windows\Globalization\MCT

This directory is hidden, even if you are showing hidden files and directories, so you need to enter into the address bar manually to get to it.

Once you are there, you should see a bunch of subdirectories, one for each theme. The names will begin with MCT- and then include a two-letter country or region code. For example, MCT-CA.

To use one of the hidden themes, navigate into the appropriate MCT directory then go into the Theme subdirectory and double click the theme file.