* 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
SetBackground(views::CreateSolidBackground(background_color_));
  
  if (menu_model_ == nullptr)
    return;
#if defined(USE_X11)
  const auto& textColor = has_focus_ ? enabled_color_ : disabled_color_;
  for (auto* child : GetChildrenInZOrder()) {
    auto* button = static_cast<SubmenuButton*>(child);
    button->SetTextColor(views::Button::STATE_NORMAL, textColor);
    button->SetTextColor(views::Button::STATE_DISABLED, disabled_color_);
    button->SetTextColor(views::Button::STATE_PRESSED, enabled_color_);
    button->SetTextColor(views::Button::STATE_HOVERED, textColor);
    button->SetUnderlineColor(textColor);
  }
#elif defined(OS_WIN)
  for (auto* child : GetChildrenInZOrder()) {
    auto* button = static_cast<SubmenuButton*>(child);
    button->SetUnderlineColor(color_utils::GetSysSkColor(COLOR_MENUTEXT));
  }
#endif
}
}