I have already added a section to the child themes codex page, about the Dark Theme option.
Not sure what you mean about creating a Codex Page, or which part of the information, I assume it would not be about the plugin?
I would not mind adding a page if I knew what part of the posts content I should use, any advise would help.
I think the plugin is good for a new user that wants just to tweak a style and does not often get involved at a deeper level, or does not want to do to much.
It could be developed into a nice plugin, creating a functions.php with empty init and after theme setup functions, and having the option to transfer template files from the themes parent.
I have another child theme that creates the color styles for a twenty ten child, I am thinking to build the child theme plugin code with these extras into the next version of the child theme.