BuddyPress.org

Put the bbPress theme files into your WordPress theme directory:

wp-content/themes/yourtheme

In yourtheme/css place the bbpress.css file
In yourtheme/bbpress, place all of the files you need to edit. Most of them (I believe):

bbpress/content-archive-forum.php
bbpress/content-archive-topic.php
bbpress/content-search.php
bbpress/content-single-forum.php
bbpress/content-single-reply.php
bbpress/content-single-topic-lead.php
bbpress/content-single-topic.php
bbpress/content-single-user.php
bbpress/content-single-view.php
bbpress/content-statistics.php
bbpress/content-topic-tag-edit.php
bbpress/feedback-logged-in.php
bbpress/feedback-no-access.php
bbpress/feedback-no-forums.php
bbpress/feedback-no-replies.php
bbpress/feedback-no-search.php
bbpress/feedback-no-topics.php
bbpress/form-anonymous.php
bbpress/form-forum.php
bbpress/form-protected.php
bbpress/form-reply-move.php
bbpress/form-reply.php
bbpress/form-search.php
bbpress/form-topic-merge.php
bbpress/form-topic-split.php
bbpress/form-topic-tag.php
bbpress/form-topic.php
bbpress/form-user-edit.php
bbpress/form-user-login.php
bbpress/form-user-lost-pass.php
bbpress/form-user-register.php
bbpress/form-user-roles.php
bbpress/loop-forums.php
bbpress/loop-mysingle-topic.php
bbpress/loop-mytopics.php
bbpress/loop-replies.php
bbpress/loop-search-forum.php
bbpress/loop-search-reply.php
bbpress/loop-search-topic.php
bbpress/loop-search.php
bbpress/loop-single-forum.php
bbpress/loop-single-reply.php
bbpress/loop-single-topic.php
bbpress/loop-topics.php
bbpress/pagination-replies.php
bbpress/pagination-search.php
bbpress/pagination-topics.php
bbpress/user-details.php
bbpress/user-favorites.php
bbpress/user-profile.php
bbpress/user-replies-created.php
bbpress/user-subscriptions.php
bbpress/user-topics-created.php

These are probably questions better asked on bbPress forum – remember although bbPress integrates with buddyPress it is a standalone plugin.

I doubt however you need to overload all the files as above.

There is a basic outline of what to do https://codex.bbpress.org/theme-compatibility/

Also this should be helpful:

https://codex.bbpress.org/theme-compatibility/step-by-step-guide-to-creating-a-custom-bbpress-theme/

You also only need to copy the templates that you want to change.