* Electron version: 1.7.5

### Expected behavior

When a theme color is no longer present the webContents did-change-theme-color event should return `null`.

### Actual behavior

The event returns `"#000000"`.

### How to reproduce

1. Log the returned color of the webContents did-change-theme-color event.
2. Go to https://electron.atom.io which has a theme color.
3. Go to https://atom.io which does not have a theme color.




atom/browser/api/atom_api_web_contents.cc
rect.size();
  
  
  
  gfx::Size bitmap_size = view_size;
  const gfx::NativeView native_view = view->GetNativeView();
  const float scale =
      display::Screen::GetScreen()->GetDisplayNearestView(native_view)
      .device_scale_factor();
  if (scale > 1.0f)
    bitmap_size = gfx::ScaleToCeiledSize(view_size, scale);
  view->CopyFromSurface(gfx::Rect(rect.origin(), view_size),
                        bitmap_size,
                        base::Bind(&OnCapturePageDone, callback),
                        kBGRA_8888_SkColorType);
}
void WebContents::OnCursorChange(const content::WebCursor& cursor) {
  content::CursorInfo info;
  cursor.GetCursorInfo(&info);
  if (cursor.IsCustom()) {
    Emit("cursor-changed", CursorTypeToString(info),
      gfx::Image::CreateFrom1xBitmap(info.custom_image),
      info.image_scale_factor,
      gfx::Size(info.custom_image.width(), info.custom_image.height()),
      info.hotspot);
  } else {
    Emit("cursor-changed", CursorTypeToString(info));
  }
}
void WebContents::SetSize(const SetSizeParams& params) {