This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
Will my skin break in future builds?
Unfortunately, yes. Every time there is a major change made to the interface, either visible or behind the scenes, your skin will have problems. When the Mozilla developers make changes like this, they increase the skin version number. This is the number that is defined in contents.rdf. Test your skin with every major release of Mozilla. If a message appears in the preferences window telling you that it's an old skin, you're going to have to look over the code to see what changed.
Developers try to keep changes like this to a minimum, but sometimes it just can't be helped.
What is an Access key?
An access key is a keyboard shortcut to access something. In the case of Skins the access key that you are defining is accessible by going to Edit->Preferences, Appearance->Themes, and then pressing a letter. C will select Classic and M will select Modern.