Inbuilt style for jquery-ui-datepicker

As far as I know, there is not style for datepicker. You have to register your own. The code then will be:

function rr_scripts() {
  wp_enqueue_script( 'jquery' );
  wp_enqueue_script( 'jquery-ui-datepicker', array( 'jquery' ) );

  wp_register_style( 'bootstrap_css', get_template_directory_uri() . '/assets/css/bootstrap.min.css' );
  wp_enqueue_style( 'bootstrap_css' ); // I'm using Twitter Bootstrap as CSS(if it matters)

  wp_register_style('jquery-ui', 'http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/base/jquery-ui.css');
  wp_enqueue_style( 'jquery-ui' );   
}

add_action( 'wp_enqueue_scripts', 'rr_scripts' );