* 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
set_ignore_menu_shortcuts(ignore);
}
void WebContents::SetAudioMuted(bool muted) {
  web_contents()->SetAudioMuted(muted);
}
bool WebContents::IsAudioMuted() {
  return web_contents()->IsAudioMuted();
}
void WebContents::Print(mate::Arguments* args) {
  PrintSettings settings = { false, false, base::string16() };
  if (args->Length() >= 1 && !args->GetNext(&settings)) {
    args->ThrowError();
    return;
  }
  auto print_view_manager_basic_ptr =
      printing::PrintViewManagerBasic::FromWebContents(web_contents());
  if (args->Length() == 2) {
    base::Callback<void(bool)> callback;
    if (!args->GetNext(&callback)) {
      args->ThrowError();
      return;
    }
    print_view_manager_basic_ptr->SetCallback(callback);
  }
  print_view_manager_basic_ptr->PrintNow(web_contents()->GetMainFrame(),
               settings.silent,
               settings.print_background,
               settings.device_name);
}
std::vector<printing::PrinterBasicInfo> WebContents::GetPrinterList() {