how to use iOS 13 darkmode for wkwebview

Assuming your question is asking how to change the colors of the HTML content you are displaying in a WKWebView based on whether light or dark mode is in effect, there is nothing you do in your app's code. All changes need to be in the CSS being used by your HTML content.

I have some local HTML files I use in a WKWebView. I was able to support dark mode by updating my css file.

Let's say you currently have a css file with the following:

body {
    background-color: white;
    color: black;
}

a:link {
    color: #0078b5;
    text-decoration: none;
}

Those are fine in light mode. To also support dark mode, you can add an @media section to your css:

@media (prefers-color-scheme: dark) {
    body {
        background-color: rgb(38,38,41);
        color: white;
    }
    a:link {
        color: #0096e2;
    }
    a:visited {
        color: #9d57df;
    }
}

When in dark mode, the colors in this @media section will override the corresponding colors defined outside the @media section.

Swift 5

For WKWebView, below code worked for me.

extension RichTextController : WKNavigationDelegate {
    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        let cssString = "@media (prefers-color-scheme: dark) {body {color: white;}a:link {color: #0096e2;}a:visited {color: #9d57df;}}"
        let jsString = "var style = document.createElement('style'); style.innerHTML = '\(cssString)'; document.head.appendChild(style);"
        webView.evaluateJavaScript(jsString, completionHandler: nil)
    }
}