It is due to The .deskthemepack is not recognized by Win7. It is specifically designed for win8 and currently is supported
by win7. If you want to successfully install this theme package, follow the steps below:
1. Download Windows 8 theme package--The .deskthemepack file(Of course, you already have it)
2. Download and install 7-Zip software(it is free).
3. Extract the theme package by select 7-Zip. Now You should get a .theme file and also a folder named DesktopBackground.
All desktop backgrounds are included in the .deskthemepack file.
4. Now you can open the folder, select Set as desktop background.
But if you want to use all of the pictures as win7 theme. You still need to do the following two-step:
5. Right-click on your windows desktop, and select "Personalize" options, then click Browse, select the folder you just extracted. Click Save changes button. Go back to Personalization window, click on Window Color, pick your own Window color and again click Save Changes button to save the new Window color.
6. Finally click on the newly created theme file and select Save theme for sharing option. Enter a name for your theme to save your theme. The theme will be saved. Now you can fully use this theme!!