I know in Windows XP it was not a trivial exercise to create your own themes. Is this task easier in Windows 7? And if so, what is involved?
Microsoft made creating and sharing themes really simple, thanks to a new file format
Read this article for an illustrated step-by-step guide to creating themes on Windows 7.
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.