Windows 8 themes are not different from Windows 7 themes, at least not when you look at them for the first time. The process for creating and customizing themes is the same in both operating systems but the theme files that result from the process have a different file extension and they are not compatible with each other. Why is that? I really wanted to learn the answer to this question and I decided to do a bit of research on my own. Here's what I have learned:
What is a Windows theme anyway? - A geek's definition
First, I would like to start by explaining what a Windows theme is, independent of the Windows version you are using. Microsoft says that "a theme is a combination of desktop background pictures, window colors, and sounds".
That's correct but it doesn't share the full picture. Therefore, I would like to provide a longer and more thorough definition. Here it goes:
"A Windows theme is an archive with a specific file extension (.themepack in Windows 7 and .deskthemepack in Windows 8). It includes the wallpapers used on the Desktop as well as the settings like: the standard Desktop icons (Computer, Network, Recycle Bin, etc) being used, visual style information applied to Windows and all desktop programs, the mouse cursors being used, the screensaver that runs when the computer is not in use and the sound scheme applied to the operating system."
When you extract the content of a Windows theme, you will find a structure similar to the one below: a DesktopBackground folder which includes all the wallpapers used on the Desktop and a file with the extension ".theme" which is similar to ".ini" configuration files used by all kinds of programs.
If you edit the ".theme" file, you will notice that it has a structure similar to what is shown in the screenshot below.
What's different about Windows 8 themes compared to Windows 7 themes?
Since a Windows theme has the same definition and role, no matter the Windows version you are using, why don't Windows 8 themes work in Windows 7?
In order to fully understand the technical differences, I have used an awesome theme we created, using the same wallpapers and almost the same settings, both in Windows 7 and Windows 8. It is a great looking theme.
After I created the theme manually, both in Windows 8 and Windows 7, I extracted each theme file and started looking for differences that would explain why Windows 8 themes are not compatible with Windows 7.
The first noticeable difference is the file extension. Windows 8 themes have the .deskthemepack file extension while Windows 7 themes have the .themepack file extension. Also, the type for each of these files has a different name: a Windows 8 theme file is named Windows Desktop Theme Pack File while in Windows 7 it is named Windows Theme Pack File.
If Windows themes have almost the same settings, they should have about the same size. As you can see in the screenshot above, the Windows 8 theme is only 1 KB smaller than the Windows 7 theme.
Extracting the contents of each theme file, revealed the same structure and files. Therefore, the next step was to open the ".theme" file included in each theme and compare the settings stored inside. Only then did I start to notice a few important differences.
First of all Windows 8 themes have a ThemeID - an identifier like those used in the Windows Registry. Windows 7 themes don't use such IDs.
Then, Windows 7 themes have two settings that seem to be gone from Windows 8 themes: TileWallpaper and WallpaperStyle - they are used to describe the position of the wallpaper on the screen.
Windows 8 themes introduce instead three new settings: MultimonBackgrounds - used when you have wallpapers compatible with multi monitor setups and PicturePosition - most probably, it replaces the two Windows 7 positioning settings mentioned earlier.
The third new setting is AutoColorization. This is another new feature of Windows 8 themes - the possibility to change the color applied to all the windows on the screen, based on the main color of the currently active wallpaper.
The list of differences stops here and it shorter than I expected.
As you can see from my little research exercise, the reason why Windows 8 themes don't work in Windows 7 is because Windows 8 introduces new features to its desktop themes: multi-monitor support, auto colorization of all opened windows based on the active wallpaper and a new identification system for each theme.