BuddyPress.org

If you are talking about the adminbar (very top) then the template plugin references the file from the bp-default theme in the buddypress plugin directory but you can disable that file and code your own styles or place a copy of the file in your themes folder and add an @import call to it in your style.css, then changes will be preserved when you update BP (that can be a good or bad thing depending on the changes)