* 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
: background_color_(kDefaultColor), window_(window) {
  RefreshColorCache();
  UpdateViewColors();
  SetLayoutManager(
      std::make_unique<views::BoxLayout>(views::BoxLayout::kHorizontal));
  window_->GetFocusManager()->AddFocusChangeListener(this);
}
MenuBar::~MenuBar() {
  window_->GetFocusManager()->RemoveFocusChangeListener(this);
}
void MenuBar::SetMenu(AtomMenuModel* model) {
  menu_model_ = model;
  RebuildChildren();
}
void MenuBar::SetAcceleratorVisibility(bool visible) {
  for (auto* child : GetChildrenInZOrder())
    static_cast<SubmenuButton*>(child)->SetAcceleratorVisibility(visible);
}
MenuBar::View* MenuBar::FindAccelChild(base::char16 key) {
  for (auto* child : GetChildrenInZOrder()) {
    if (static_cast<SubmenuButton*>(child)->accelerator() == key)
      return child;
  }
  return nullptr;
}
bool MenuBar::HasAccelerator(base::char16 key) {