How to reference a stylesheet from an HTML file in an iOS project?

I have HTML files in a folder 'Hyms' in an iOS project. I also have a stylesheet 'style.css' in the same folder. I am failing to reference the stylesheet from the .html file

I have tried using a method to apply a stylesheet:

func insertCSSString(into webView: WKWebView) {
        let cssString = "body { font-size: 50px; color: #f00 }"
        let jsString = "var style = document.createElement('style'); style.innerHTML = '\(cssString)'; document.head.appendChild(style);"
        webView.evaluateJavaScript(jsString, completionHandler: nil)
    }