BuddyPress.org

When you refer to a child theme it refers to the ability to create a new blank folder within the ‘theme’ folder this would be considered a ‘Child theme’ if it simply had a stylesheet residing in it’s root /top level that stated it was using the files from a particular parent theme, what then happens is having selected this theme – which now would appear in your dashboard themes page, – WP examines the stylesheet information at the top of the file and finds that it instructs WP to look for files in a particular theme as being the main theme files to use; however the clever trick is that WP first knows that it must look in your child theme for necessary files FIRST! and if they are not found THEN look for the necessary file in the stated parent theme, thus you do not have to have any files in your child theme as WP will use the parent files, but you can add new CSS rulesets to the stylesheet living in the child theme and these will be read and acted on (that stylesheet references the parent CSS stylesheet files through the @import rule first and then reads on for any additional rules you have added below the @import calls)

If you do not want to make any changes to BP-Default theme then you simply activate that theme as seen in the theme page in the dashboard. When members create a blog then those blogs should use the twentyten theme or whatever other themes you choose to download and make available (Note that you decide what themes are available to members via the ‘Super Admin >Themes’ page only the themes that you select ‘yes’ to are available to member blogs so to avoid them attempting to use the BP-Default on user blogs you ensure BP themes are not selected)

If you decide that you do need to make changes to BP-Default theme then you would create a folder under ‘Themes’ name it ? ‘my-bp-default’
to the folder you add the requisite stylesheet and now you can move files as required from bp-Default original folder to your new folder and edit them as you wish, or simply create new rulesets to style elements which you place in the new stylesheet file. BP/WP now looks at your new child theme for any files that it must use in preference to the same versions that might reside in the original theme folder.

Apologies if the above covers ground you already understand.