There is only one file type that uses the .THEMEPACK extension:
Windows 7 Theme Pack
System theme created for the Windows 7 operating system; saved in the .CAB format and includes desktop background images and interface customizations for window appearance, icons, cursors, sounds, screen savers, etc; can be saved and loaded using the Personalization Control Panel in Windows 7.
Windows themes may also be saved as .THEME files. However, THEME files use a plain text format and cannot include packaged graphics, sounds, and other theme files. The THEMEPACK file format was introduced with Windows 7 to help users share themes.
Because THEMEPACK files are stored in the CAB format, you can open them with any tool that support CAB files to view their contents manually. Such tools include 7-Zip and WinZip.
Windows 8 and 10 can import Windows 7 THEMEPACK files through "Themes" in "Windows Settings." However, Windows 8 saves themes in a different format with a .DESKTHEMEPACK extension. These files are not backward-compatible with Windows 7.
|Microsoft Windows Personalization Control Panel|
Updated: August 24, 2012