Apply TwentyFifteen Navigation as a Custom Menu

I’d like to apply the same dynamic menu that’s used in WP’s Twenty Fifteen theme to a new menu that I’m using in a different WP theme (Storefront).

I’ve registered a new menu using this code:

function register_my_menu() {
  register_nav_menu('new-menu',__( 'New Menu' ));
}
add_action( 'init', 'register_my_menu' );

…and the menu works and shows up fine (see ).

Basically, I’d like to have the tree structure for my new menu to be collapsible, as it is in the Twenty Fifteen theme.

Any help (and code details) would be greatly appreciated!