Injecting a stylesheet into a WKWebView

exampleScript.js

// Any website loaded by the WebView can post to the webkit message handler named "myMessageHandler"
// by calling this JavaScript API. It's available for use immediately; no need to wait for document load or anything.
webkit.messageHandlers.myMessageHandler.postMessage("hello world");

ViewController.swift

import WebKit
import UIKit

class ViewController: UIViewController {
    let webView: WKWebView = WKWebView(frame: self.view.frame, configuration: WKWebViewConfiguration())
    
    override func loadView() {
        super.loadView()

        self.view = webView
        let userScript: WKUserScript = WKUserScript(
            source:
"""
(function injectWebkitAppRegionStyle(){
    const styleEle = document.createElement('style');
    styleEle.type = 'text/css';
    styleEle.innerHTML = 'body { -webkit-app-region: drag; }';
    document.head.appendChild(styleEle);
})();
"""
            injectionTime: WKUserScriptInjectionTime.atDocumentStart,
            forMainFrameOnly: true
        )
        self.webView.configuration.userContentController.addUserScript(userScript)
        // Set up a bridge called "myMessageHandler" by which any website in the WebView can post messages to the Swift app
        self.webView.configuration.userContentController.add(self, name: "myMessageHandler")

        guard let url = URL(string: "https://birchlabs.co.uk") else { return; }
        self.webView.load(URLRequest(url: url))
    }
}

extension ViewController: WKNavigationDelegate {
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) -> Void {
        // We check message.name because you can register various different userContentControllers with different names
        if(message.name == "myMessageHandler") {
            // message.body holds the payload.
            // The website could, for example, post a stringified JSON message and here you could
            // decode it into a Swift Dictionary. But for now, we're just posting "hello world".
            print("Message from the WebView: \(message.body)");
            return
        }
    }
}