How to enqueue the style using wp_enqueue_style()

This is what you could do:

1 – Put the CSS in a separate file and save it in your theme directory.
2 – Add the following code in your functions php:

function wpse_89494_enqueue_scripts() {
  if ( has_nav_menu( 'secondary' ) ) {
    wp_enqueue_style( 
      'wpse_89494_style_1', 
      get_template_directory_uri() . '/your-style_1.css' 
    );
  }
  if ( has_nav_menu( 'primary' ) ) {
    wp_enqueue_style( 
      'wpse_89494_style_2', 
      get_template_directory_uri() . '/your-style_2.css' 
    );
  }
  if ( ! has_nav_menu( 'primary' ) && ! has_nav_menu( 'secondary' ) ) {
    wp_enqueue_style( 
      'wpse_89494_style_3', 
      get_template_directory_uri() . '/your-style_3.css' 
    );
  }
}

add_action( 'wp_enqueue_scripts', 'wpse_89494_enqueue_scripts' );