Secondary Menu

Hello, I was wondering how to add a secondary menu to this theme?

In functions.php I have added 'second-menu' => __( 'Second Menu' ) as below:

register_nav_menus( array(
         'primary' => esc_html__( 'Primary Menu', 'quark' ),
         'second-menu' => __( 'Second Menu' )
			) );

However that only gives me the checkbox.
How do I get this theme to allow me to build a secondary menu in the dashboard?. I would like to have it placed in a widget.

Thanks,

Alex.