* 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
content::SavePageType* out) {
    std::string save_type;
    if (!ConvertFromV8(isolate, val, &save_type))
      return false;
    save_type = base::ToLowerASCII(save_type);
    if (save_type == "htmlonly") {
      *out = content::SAVE_PAGE_TYPE_AS_ONLY_HTML;
    } else if (save_type == "htmlcomplete") {
      *out = content::SAVE_PAGE_TYPE_AS_COMPLETE_HTML;
    } else if (save_type == "mhtml") {
      *out = content::SAVE_PAGE_TYPE_AS_MHTML;
    } else {
      return false;
    }
    return true;
  }
};
template<>
struct Converter<atom::api::WebContents::Type> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,