Re: Blog Theme / Buddypress Theme
Yes, you’ll have to either modify the CSS for the BuddyPress page templates, and / or actually modify the template files (PHP / HTML), depending on how much you want to change the default layout.
Don’t make the changes in bp-themes/bp-default though. Instead, it is best to create your own child theme: https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/
In the standard setup, the site administrator has to install the themes that will be available to the users for their own blogs. I don’t know if this can be overridden to allow the users to install their own themes directly.
