fix flickering background when entering the connectiviy view

deltachat-ios/Controller/ConnectivityViewController.swift

@@ -17,6 +17,9 @@ class ConnectivityViewController: WebViewViewController {
     override func viewDidLoad() {
         super.viewDidLoad()
         self.title = String.localized("connectivity")
+        self.webView.isOpaque = false
+        self.webView.backgroundColor = .clear
+        view.backgroundColor = DcColors.defaultBackgroundColor
         loadHtml()
     }