I've just download an official Windows 7 theme here
But I cannot install it after download - double clicking will say unknown file type !?
I may be wrong, but I don't think that this will work on Windows 7. It is a dual screen wallpaper pack which is only supported by Windows 8. Try this one: African Wildlife Theme. If it works then you simply can't use deskthemepacks. Just themepacks. Hover over the download button to see the filetype.
Looks like 7zip is the answer.
The only reason you are not able to use it on windows 7 is because it not for windows 7.
Quoting the website :