Okay, let me see if I can explain what I want to do.
I would like to have one main website/blog with a main/general theme and only one (buddypress) community section.
I would like this main website to have several niche (related) websites linked off of it.
Users would/could be members of the main site/blog and have access to all other sites/blogs with that one login/password.
I want to mange this all in one dashboard without having to login and out of multiple websites/dashboards.
I want the ability to have a different header and sidebar for each of the niche websites.
It would be nice if the niche websites could have their own independent urls, but that's not completely necessary. I would be okay with http://www.abc/123, http://www.abc/456 etc. so long as each site could have it's own independent header and sidebar.
Does that make sense? Is there a theme that would be better suited to this? I'd like to keep things as simple as possible....if at all possible.
I'm looking for something along the lines of (but not quite) http://www.apartmenttherapy.com/