By this I mean, if user visits a certain page, then that page (and only that page) is shown in the theme the admin specifies.
Another good thing with regards to the page system would be sectioning of pages. Like for instance if you wanted a section that was about different types of stones, you would have:
http://eocms.com/stones
and then
http://eocms.com/stones/pebbles.html
http://eocms.com/stones/bricks.html

all of which could have the same theme.