I hope i am wrong with this but it appears that you can only run one theme on the new setup. I say that because you have to select the theme for the CMS via the pencil on the frontpage to edit it. So whatever them my members are running they will always see the same frontpage which is very limiting.
Did you do what I suggested and go look at what is selected under "style" for the Section? If you have it set to a particular style, then no matter what you change the style chooser to, it won't change. Inherit means it always gets what it's parent is set to (so if it's set to a particular style, then so is it). Use Default means you may change it in the style chooser and it will show.