This project should provide a theme-only package. Most of users I believe will just install the theme on an already configured SquirrelMail, and just overriding all file with this package is not advisable at best, since it will erase existing config.php files and possible others.
I tried just including theme files and including Outlook Skin as one theme on the list, but it didn't work. I'm missing something, and I don't want to further search and edit my config files.
Since I believe most users will fit in my case, I advise offering a theme-only package for existing SM users.