Theme_Upgrader::upgrade( string $theme, array $args = array() )

	public function upgrade( $theme, $args = array() ) {

		$defaults = array(
			'clear_update_cache' => true,
		);
		$parsed_args = wp_parse_args( $args, $defaults );

		$this->init();
		$this->upgrade_strings();

		// Is an update available?
		$current = get_site_transient( 'update_themes' );
		if ( !isset( $current->response[ $theme ] ) ) {
			$this->skin->before();
			$this->skin->set_result(false);
			$this->skin->error( 'up_to_date' );
			$this->skin->after();
			return false;
		}

		$r = $current->response[ $theme ];

		add_filter('upgrader_pre_install', array($this, 'current_before'), 10, 2);
		add_filter('upgrader_post_install', array($this, 'current_after'), 10, 2);
		add_filter('upgrader_clear_destination', array($this, 'delete_old_theme'), 10, 4);
		if ( $parsed_args['clear_update_cache'] ) {
			// Clear cache so wp_update_themes() knows about the new theme.
			add_action( 'upgrader_process_complete', 'wp_clean_themes_cache', 9, 0 );
		}

		$this->run( array(
			'package' => $r['package'],
			'destination' => get_theme_root( $theme ),
			'clear_destination' => true,
			'clear_working' => true,
			'hook_extra' => array(
				'theme' => $theme,
				'type' => 'theme',
				'action' => 'update',
			),
		) );

		remove_action( 'upgrader_process_complete', 'wp_clean_themes_cache', 9 );
		remove_filter('upgrader_pre_install', array($this, 'current_before'));
		remove_filter('upgrader_post_install', array($this, 'current_after'));
		remove_filter('upgrader_clear_destination', array($this, 'delete_old_theme'));

		if ( ! $this->result || is_wp_error($this->result) )
			return $this->result;

		wp_clean_themes_cache( $parsed_args['clear_update_cache'] );

		return true;
	}