The list of forums we support is under product details and on the before you buy forum as well. phpbb 3 is not listed there.
Your stylesheet does not show because your forum path is wrong
http://www.naturemoms.com/natureforu.../subSilver.css
would it not be this
http://www.naturemoms.com/natureforu.../subSilver.css
We do not support phpbb 3.0 so not much I can suggest except going back to phpbb 2.0. Most users as part of their site maintenance take backups and that is my suggestion use your backups and download phpbb2 and restore your site to a forum we support.