* 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
} else {
    const mate::Dictionary options = mate::Dictionary::CreateEmpty(isolate);
    auto session = Session::CreateFrom(isolate, GetBrowserContext());
    session_.Reset(isolate, session.ToV8());
    InitWithSessionAndOptions(isolate, web_contents, session, options);
  }
}
WebContents::WebContents(v8::Isolate* isolate, const mate::Dictionary& options)
    : embedder_(nullptr),
      zoom_controller_(nullptr),
      type_(BROWSER_WINDOW),
      request_id_(0),
      background_throttling_(true),
      enable_devtools_(true) {
  
  
  MarkHighMemoryUsage();
  
  options.Get("backgroundThrottling", &background_throttling_);
  
  
  
  
  
  bool b = false;
  if (options.Get("isGuest", &b) && b)
    type_ = WEB_VIEW;
  else if (options.Get("isBackgroundPage", &b) && b)
    type_ = BACKGROUND_PAGE;
  else if (options.Get("isBrowserView", &b) && b)