* 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
RefreshColorCache(theme);
  UpdateViewColors();
}
void MenuBar::OnDidChangeFocus(View* focused_before, View* focused_now) {
  
  const auto had_focus = has_focus_;
  has_focus_ = focused_now != nullptr;
  if (has_focus_ != had_focus)
    UpdateViewColors();
}
void MenuBar::RebuildChildren() {
  RemoveAllChildViews(true);
  for (int i = 0, n = GetItemCount(); i < n; ++i) {
    auto* button =
        new SubmenuButton(menu_model_->GetLabelAt(i), this, background_color_);
    button->set_tag(i);
    AddChildView(button);
  }
  UpdateViewColors();
}
void MenuBar::UpdateViewColors() {