BuddyPress.org

function change_template( $blog_id )

{

switch_to_blog( $blog_id );

update_option('template', 'bp-sn-parent');

update_option('stylesheet', 'YOUR_TEMPLATE_NAME');

}

add_action( 'wpmu_new_blog', 'change_template' );

put the above code into your bp-custom.php file and it’ll use the template you specified instead of YOUR_TEMPLATE_NAME.