Download the .themepack file, double-click on it and Windows will automatically install and apply it.
PS: You can also download other official Windows 7 themes, Desktop gadgets and wallpapers from Microsoft using following steps:
1. Right-click on Desktop and select "Personalize". Now click on "Get more themes online" link:
2. It’ll open following webpage in your browser:
Alternatively you can directly open the above URL in your browser.
3. From the above webpage, you can download various free themes, gadgets and wallpapers for Windows 7.