look into functions.php, how the parent theme adds the .singular
class to the body_class for certain templates:
function twentyeleven_body_classes( $classes ) {
if ( function_exists( 'is_multi_author' ) && ! is_multi_author() )
$classes[] = 'single-author';
if ( is_singular() && ! is_home() && ! is_page_template( 'showcase.php' ) && ! is_page_template( 'sidebar-page.php' ) )
$classes[] = 'singular';
return $classes;
}
add_filter( 'body_class', 'twentyeleven_body_classes' );
you should be able to replicate this for your child theme and the custom archive templates.
Thanks very much- this was what I needed. 🙂