First, configure your windows 7 theme the way you want to share it.
Save your theme, then right click on the saved theme’s name and choose “Save theme for sharing”.
Download the Microsoft Cabinet Software Development Kit @ http://support.microsoft.com/kb/310618
Extract the files to a useful location. You may want to make life easier and just copy the CABINET.DLL and CABARC.exe to a folder at the root of your C drive (I tossed them in a folder I called ‘cab’).
Next, find the saved version of your theme, rename it from .themepack to .cab and extract the contents using your preferred compression utility.
Once you’ve done that, you can tweak the theme information by editing the .theme file in Notepad. You can add a logo, etc. using the documentation provided by Microsoft @ http://msdn.microsoft.com/en-us/library/bb773190(VS.85).aspx
Once you’ve done that, it is time to add a logo (if you want), and according to the documentation is must be a .png file that is scaled to 80×240 pixels. Toss it into the same folder as the .theme and the folder containing your desktop backgrounds.
Now you’re set to compress the folder back into a cab file. This is the tricky part.
For me, I created a theme for Mass Effect 2 using a few of the images from the official Bioware website and just changed the tint to aero. So, I tossed my me2 folder onto the root of the C drive. I next opened a CMD prompt and ran the following (without quotes) “C:\cab\cabarc -r -p -P me2\ n me2.cab c:\me2\*.*”
This told cabarc to check sub-directories (-r), to preserve path names (-p) and to strip out the prefix before adding the files (-P) meaning the me2 directory itself wouldn’t be created to store the files for the theme.
Voila, you now have a .cab file you can now rename to a .themepack and a working Windows 7 Theme.