BuddyPress.org

@johnwinch42 – I think it is a terminology problem:

Each page in a WordPress / BuddyPress site can have it’s own template, so for example a template for posts (single.php), for categories (archive.php), etc.. You can even specialise it further, so that different types of posts have different templates and so on.

BuddyPress extends WordPress in the same way, adding new template files for groups, activity, members, profiles, etc.

A theme is a collection of template files, CSS, and javascripts, that are used together to give your site its design, look and feel.

A BuddyPress site typically has one theme that contains the template files for both the main blog and the social network parts. Although it’s one theme, you can style each part uniquely (e.g. the main blog and social network can look different), because you have different templates within that theme for each part of the site.

Take a look at the BuddyPress default theme (it’s in plugins/buddypress/bp-themes/bp-default), and you will see all the separate template files, including sub-folders with the templates for activity, members, etc.

Your secondary blogs can have any WordPress theme you want: you simply install more themes, and assign them on a blog by blog basis to the other blogs.