On Windows 7, 8, and Server 2012, themes can be downloaded from Microsoft here and are typically installed on a per user basis in the following directory:
%localappdata%\Microsoft\Windows\Themes
To install a .themepack file for all users, it requires a bit of manual intervention. Today when searching for the answer, I found this perfect blog article that explains exactly how to do it so there’s no real point in re-writing it all verbatim here.
Basically you have to rename the themepack file to a cab file, extract it’s contents to a folder, create another folder called “DesktopBackground” and move all the pictures into it, edit the paths to the new location in the .theme file, and then finally move both the folder and the theme file to the follow directory:
%SystemRoot%\Resources\Themes