BuddyPress.org

The old theme (BP Classic) is available in the BP Backwards Compatibility plugin:

https://wordpress.org/extend/plugins/buddypress-backwards-compatibility/

The new theme is built with child themes in mind! So yes, you can use a custom child theme on top of bp-default!

More info here:

https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/