* 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
PrintSettings* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    dict.Get("silent", &(out->silent));
    dict.Get("printBackground", &(out->print_background));
    dict.Get("deviceName", &(out->device_name));
    return true;
  }
};
template<>
struct Converter<printing::PrinterBasicInfo> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,
                                   const printing::PrinterBasicInfo& val) {
    mate::Dictionary dict(isolate, v8::Object::New(isolate));
    dict.Set("name", val.printer_name);
    dict.Set("description", val.printer_description);
    dict.Set("status", val.printer_status);
    dict.Set("isDefault", val.is_default ? true : false);
    dict.Set("options", val.options);
    return dict.GetHandle();
  }
};
template<>
struct Converter<WindowOpenDisposition> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,