While installing Windows 7, Based on the time zone you select the corresponding theme would be enabled and installed.
If you want to see what are all the other themes which is present along with the current themes. Here is the way
All the Windows7 hidden theme packs can be found under C:\Windows\Globalization\MCT directory. The MCT directory is hidden and enabling the setting for showing hidden folders won’t reveal the folder’s appearance.
Hence, to access the MCT folder, enter the path C:\Windows\Globalization\MCT in Windows explorer or Run command window and you will see many theme folders as shown below:
and to use a theme, open the theme sub directory and double click the .theme file. This will add the theme in the “Personalize” option and then you can start using a hidden theme in Windows7.