* 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 FindAccelChild(key) != nullptr;
}
void MenuBar::ActivateAccelerator(base::char16 key) {
  auto* child = FindAccelChild(key);
  if (child)
    static_cast<SubmenuButton*>(child)->Activate(nullptr);
}
int MenuBar::GetItemCount() const {
  return menu_model_ ? menu_model_->GetItemCount() : 0;
}
bool MenuBar::GetMenuButtonFromScreenPoint(const gfx::Point& screenPoint,
                                           AtomMenuModel** menu_model,
                                           views::MenuButton** button) {
  if (!GetBoundsInScreen().Contains(screenPoint))
    return false;
  auto children = GetChildrenInZOrder();
  for (int i = 0, n = children.size(); i < n; ++i) {
    if (children[i]->GetBoundsInScreen().Contains(screenPoint) &&
        (menu_model_->GetTypeAt(i) == AtomMenuModel::TYPE_SUBMENU)) {
      *menu_model = menu_model_->GetSubmenuModelAt(i);
      *button = static_cast<views::MenuButton*>(children[i]);
      return true;
    }
  }
  return false;
}
const char* MenuBar::GetClassName() const {