Windows 7-8 Themes / Skins
This is a simple thing and only thing is to do is open it and it will do reset .. If Windows is not recognizing *.themepack files use this method and try again.
1) Press WinKey+R > type regedit and press Enter
2) Navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.themepack
3) Make sure there is no UserChoice key (subfolder)
Give a try :D there is almost 480 themes.
Windows 7 - 8 Themes
How to download -(First you must have a torrent downloader install , click on that link)