* 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
#include "atom/browser/ui/views/menu_bar.h"
#include <memory>
#include "atom/browser/ui/views/menu_delegate.h"
#include "atom/browser/ui/views/submenu_button.h"
#include "ui/base/models/menu_model.h"
#include "ui/views/background.h"
#include "ui/views/layout/box_layout.h"
#if defined(OS_WIN)
#include "ui/gfx/color_utils.h"
#endif
namespace atom {
namespace {
const SkColor kDefaultColor = SkColorSetARGB(255, 233, 233, 233);
}  
const char MenuBar::kViewClassName[] = "ElectronMenuBar";
MenuBar::MenuBar(views::View* window)