* Electron version: 1.7.5

### Expected behavior

When a theme color is no longer present the webContents did-change-theme-color event should return `null`.

### Actual behavior

The event returns `"#000000"`.

### How to reproduce

1. Log the returned color of the webContents did-change-theme-color event.
2. Go to https://electron.atom.io which has a theme color.
3. Go to https://atom.io which does not have a theme color.




atom/browser/api/atom_api_web_contents.cc
web_contents()->FocusThroughTabTraversal(reverse);
}
bool WebContents::SendIPCMessage(bool all_frames,
                                 const base::string16& channel,
                                 const base::ListValue& args) {
  return Send(new AtomViewMsg_Message(routing_id(), all_frames, channel, args));
}
void WebContents::SendInputEvent(v8::Isolate* isolate,
                                 v8::Local<v8::Value> input_event) {
  const auto view = static_cast<content::RenderWidgetHostViewBase*>(
    web_contents()->GetRenderWidgetHostView());
  if (!view)
    return;
  int type = mate::GetWebInputEventType(isolate, input_event);
  if (blink::WebInputEvent::IsMouseEventType(type)) {
    blink::WebMouseEvent mouse_event;
    if (mate::ConvertFromV8(isolate, input_event, &mouse_event)) {
      view->ProcessMouseEvent(mouse_event, ui::LatencyInfo());
      return;
    }
  } else if (blink::WebInputEvent::IsKeyboardEventType(type)) {
    content::NativeWebKeyboardEvent keyboard_event(
        blink::WebKeyboardEvent::kRawKeyDown,
        blink::WebInputEvent::kNoModifiers,
        ui::EventTimeForNow());