* 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
std::vector<printing::PrinterBasicInfo> printers;
  auto print_backend = printing::PrintBackend::CreateInstance(nullptr);
  print_backend->EnumeratePrinters(&printers);
  return printers;
}
void WebContents::PrintToPDF(const base::DictionaryValue& setting,
                             const PrintToPDFCallback& callback) {
  printing::PrintPreviewMessageHandler::FromWebContents(web_contents())->
      PrintToPDF(setting, callback);
}
void WebContents::AddWorkSpace(mate::Arguments* args,
                               const base::FilePath& path) {
  if (path.empty()) {
    args->ThrowError("path cannot be empty");
    return;
  }
  DevToolsAddFileSystem(path);
}
void WebContents::RemoveWorkSpace(mate::Arguments* args,