Sunset and Sunrise are the most beautiful daily occurring phenomena of the nature which everyone can see but no one cares. Wallpapers of the same are one of the most loved ones. Here is a free Windows 7 theme named as Sunsets Sunrises whic has 4 beautiful wallpapers.
The Sunsets Sunrises Windows 7 theme can be downloaded from Microsoft Official Site here.
This is an official theme pack and hence it can be applied directly by downloading and double clicking it.