Provide your own CSS to a WKWebView instance, even if it's loaded from a nib or storyboard.

WebViewCSSExtension.swift

extension WKWebView {
  
  //via https://stackoverflow.com/a/53706678
  //and https://stackoverflow.com/a/53141055
  
    func applyStylesheet() {
        guard
          let path = Bundle.main.path(forResource: "style", ofType: "css"), //load style.css from your app bundle
          let cssString = try? String(contentsOfFile: path).components(separatedBy: .newlines).joined()
        else {
          return
        }

        let source = """
           var style = document.createElement('style');
           style.innerHTML = '\(cssString)';
           document.head.appendChild(style);
        """

        let userScript = WKUserScript(source: source,
                                      injectionTime: .atDocumentEnd,
                                      forMainFrameOnly: true)
        
        self.configuration.userContentController.addUserScript(userScript)
    }
}