// Enable Custom Logo
      add_theme_support( 'custom-logo', array(
          'height'      => 200,
          'width'       => 400,
          'flex-width' => true,
      ) );

    $old_logo = get_theme_mod( 'header_logo' );
    if ( $old_logo ) {

      wp_enqueue_style( 'wpse_89494_style_3', get_template_directory_uri() . '/your-style_3.css' );

    }

if ( ! has_custom_logo() ) {
    // Enqueue some google fonts
    wp_enqueue_style( 'google-fonts', 'https://fonts.googleapis.com/css?family=Roboto:400' );
}