Creating Themes in Windows 7

Solution 1:

Microsoft made creating and sharing themes really simple, thanks to a new file format .themepack, which is basically a special ZIP format that includes cursors, sounds, wallpapers, icons, colors and more.

Read this article for an illustrated step-by-step guide to creating themes on Windows 7.

Solution 2:

If you actually meant hacking .msstyle files, it's just as hard in Windows 7.

I think you will need to open the aero.msstyle file up using XN Resource Editor and modify the files in Photoshop. Or you could modify a custom theme that somebody else has made. You can find them, and other people who have done this, on Deviantart.

Personally, I don't think many amateur themes are ever going to look as good as the default Aero styles, although there may be some good ones that are just slightly altered.