BuddyPress.org

Template pack walkthrough might help you
https://codex.buddypress.org/theme-development/bp-template-pack-walkthrough-level-easy-2/

You might also want to see how someone created a BP Arras child theme. https://buddypress.org/community/groups/arras-theme-children-for-buddypress/forum/topic/brownbuddy-arras-child-theme-for-buddypress