If you feel that the themes (by default has 4 or 5 different themes included on installation) is not enough for personalization, Windows 7 (from build 6956 onwards, including build 7000) has hidden themes that can be unlocked, and applied as the theme for Windows 7 system. These locked hidden themes is actually meant for some regional countries such as Australia, Canada, Great Britain, United States and South Africa.

Original number of themes included in Windows 7
To unlock and reveal the hidden regional themes in Windows 7, do the following:
1. Open Windows Explorer
2. Click
Organize, and select
Folder and Search Options.
3. Go to View tab.
4. Select Show hidden files, folders and drivers and uncheck
Hide protected operating system files (Recommended). If prompted with confirmation, click
Yes.
5. Click
OK.
6. Browse to the following folder:
\Windows\Globalization\MCT\
7. There are five folders in the name with format
MCT-XX (where XX is
AU, CA, GB, US, or ZA) which represents globalization settings for each region. Go into the folder that you want to activate its theme.
Note: AU, CA and ZA regions have the same themes.
8. Open the
Theme folder inside the selected region folder.
9. Double click on the
XX.theme file to apply the theme to the Windows 7 desktop system. Once a theme is ran and activated, the theme will be remembered and saved into Personalization options, so that user can change or select the theme again directly from Personalization settings screen.
10. Now to go to
Folder Options to reverse the first 5 steps to hide the hidden and protected system files and folders again.

After unlocking, there are more themes for personalize selection.
Alternatively, go to
\Windows\winsxs, and search for
*.theme. Double click on the first 5 themes with name in the format of XX as listed above to apply and save the theme.
ENJOY!