如何使用WKWebView对xls文件应用深色模式？

import WebKit

let webView = WKWebView(frame: CGRect(x: 0, y: 0, width: view.frame.width, height: view.frame.height))
view.addSubview(webView)

if let url = Bundle.main.url(forResource: "example", withExtension: "xls") {
    let request = URLRequest(url: url)
    webView.load(request)
}

webView.navigationDelegate = self

extension ViewController: WKNavigationDelegate {
    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        applyDarkMode()
    }
}

func applyDarkMode() {
    let darkModeScript = """
    var style = document.createElement('style');
    style.innerHTML = 'body { background-color: #000000; color: #ffffff; }';
    document.head.appendChild(style);
    """
    webView.evaluateJavaScript(darkModeScript, completionHandler: nil)
}