I know it is an old question but im pasting this here in case someone is looking for the same answer.
Overloading Template Compatibility theme files
Template compatibility also runs a check to see if two directories or folders exist in a theme:
If either of these two folders exist in your theme and they contain BP template files then those files will be used in preference to the bp plugins versions.
Therefore, you can modify any bp theme compatibility template by copying it over from:
/my-theme/community/ or /my-theme/buddypress/
N.B. Inside the subfolder
‘community’ you must preserve the path structure/folders that exist in the BP original /buddypress/ folder so /activity/ must be created to hold index.php or any of the other activity templates.
Additionally to keep things neat & tidy you can keep your custom parent template file ‘community.php’ in these folders as well rather than your theme root.
You may override the css by adding a folder /css/* to your theme root if you then, either, copy buddypress.css from /bp-legacy/ or create a new file named buddypress.css this file will be used instead of the buddypress version.
* As of BP 1.8 the paths for assets i.e styles and JS has been modified to look to your ‘buddypress’ or ‘community’ folders first, this means you will be able to locate your /css/ folder inside your buddypress one.