* 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
if (embedder_ && embedder_->IsOffScreen()) {
      auto* view = new OffScreenWebContentsView(false,
          base::Bind(&WebContents::OnPaint, base::Unretained(this)));
      params.view = view;
      params.delegate_view = view;
      web_contents = content::WebContents::Create(params);
      view->SetWebContents(web_contents);
    } else {
#endif
      web_contents = content::WebContents::Create(params);
#if defined(ENABLE_OSR)
    }
  } else if (IsOffScreen()) {
    bool transparent = false;
    options.Get("transparent", &transparent);
    content::WebContents::CreateParams params(session->browser_context());
    auto* view = new OffScreenWebContentsView(
        transparent, base::Bind(&WebContents::OnPaint, base::Unretained(this)));
    params.view = view;
    params.delegate_view = view;
    web_contents = content::WebContents::Create(params);
    view->SetWebContents(web_contents);
#endif
  } else {
    content::WebContents::CreateParams params(session->browser_context());
    web_contents = content::WebContents::Create(params);
  }
  InitWithSessionAndOptions(isolate, web_contents, session, options);
}
void WebContents::InitWithSessionAndOptions(v8::Isolate* isolate,