* 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
const SkBitmap& bitmap,
                       content::ReadbackResponse response) {
  callback.Run(gfx::Image::CreateFrom1xBitmap(bitmap));
}
}  
WebContents::WebContents(v8::Isolate* isolate,
                         content::WebContents* web_contents,
                         Type type)
    : content::WebContentsObserver(web_contents),
      embedder_(nullptr),
      zoom_controller_(nullptr),
      type_(type),
      request_id_(0),
      background_throttling_(true),
      enable_devtools_(true) {
  if (type == REMOTE) {
    web_contents->SetUserAgentOverride(GetBrowserContext()->GetUserAgent());
    Init(isolate);
    AttachAsUserData(web_contents);