如何在Swift4的GooglePlacePicker中自定义导航栏？

import UIKit
import GooglePlaces

class CustomNavigationController: UINavigationController, GMSPlacePickerViewControllerDelegate {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 设置导航栏的背景颜色
        navigationBar.barTintColor = UIColor.blue
        
        // 设置导航栏标题颜色
        navigationBar.titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.white]
        
        // 添加一个自定义的返回按钮
        let backButton = UIBarButtonItem(title: "返回", style: .plain, target: self, action: #selector(backButtonTapped))
        navigationItem.leftBarButtonItem = backButton
    }
    
    @objc func backButtonTapped() {
        // 执行返回操作
        popViewController(animated: true)
    }
    
    func showPlacePicker() {
        let config = GMSPlacePickerConfig(viewport: nil)
        let placePicker = GMSPlacePickerViewController(config: config)
        placePicker.delegate = self
        
        // 设置导航栏的标题
        placePicker.navigationItem.title = "选择地点"
        
        // 设置导航栏的样式
        placePicker.navigationController?.navigationBar.barTintColor = UIColor.blue
        placePicker.navigationController?.navigationBar.titleTextAttributes = [NSAttributedString.Key.foregroundColor: UIColor.white]
        
        present(placePicker, animated: true, completion: nil)
    }
    
    // 实现GMSPlacePickerViewControllerDelegate的相关方法
    func placePicker(_ viewController: GMSPlacePickerViewController, didPick place: GMSPlace) {
        // 在选择地点后执行相应的操作
        // ...
        
        // 返回上一个视图控制器
        dismiss(animated: true, completion: nil)
    }
    
    func placePickerDidCancel(_ viewController: GMSPlacePickerViewController) {
        // 用户取消选择地点
        dismiss(animated: true, completion: nil)
    }
}