* Electron Version: 2.x, 3.x
* Operating System (Platform and Version): Linux
* Last known working Electron version:  1.8

**Expected Behavior**
I would expect the menubar's background color to use a background based on the GTK theme. Up to Electron 2.0 this used to work (albeit only for the light theme variant), see for example the screenshot below:

![screenshot from 2018-06-22 11-20-21](https://user-images.githubusercontent.com/325102/41769175-b76bc922-760e-11e8-8b55-d0f7cc20fe0c.png)

**Actual behavior**
In Electron 2.x and 3.x the menubar is always white. Using both the light and dark theme variant. 

![screenshot from 2018-06-22 11-19-32](https://user-images.githubusercontent.com/325102/41769298-1a69ad1e-760f-11e8-8643-99828bc77779.png)

![screenshot from 2018-06-22 11-19-55](https://user-images.githubusercontent.com/325102/41769301-1fb9017a-760f-11e8-9993-6f479ffef826.png)

**To Reproduce**
This is reproducible for me with Electron Quick Start. Is it supposed to be this way or might this be caused by my Desktop configuration?

atom/browser/ui/views/menu_bar.cc
return kViewClassName;
}
void MenuBar::OnMenuButtonClicked(views::MenuButton* source,
                                  const gfx::Point& point,
                                  const ui::Event* event) {
  
  SetAcceleratorVisibility(false);
  if (!menu_model_)
    return;
  if (!window_->HasFocus())
    window_->RequestFocus();
  int id = source->tag();
  AtomMenuModel::ItemType type = menu_model_->GetTypeAt(id);
  if (type != AtomMenuModel::TYPE_SUBMENU) {
    menu_model_->ActivatedAt(id, 0);
    return;
  }
  
  MenuDelegate* menu_delegate = new MenuDelegate(this);
  menu_delegate->RunMenu(menu_model_->GetSubmenuModelAt(id), source);
}
void MenuBar::RefreshColorCache(const ui::NativeTheme* theme) {