* 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
atom::api::WebContents::Type val) {
    using Type = atom::api::WebContents::Type;
    std::string type = "";
    switch (val) {
      case Type::BACKGROUND_PAGE: type = "backgroundPage"; break;
      case Type::BROWSER_WINDOW: type = "window"; break;
      case Type::BROWSER_VIEW: type = "browserView"; break;
      case Type::REMOTE: type = "remote"; break;
      case Type::WEB_VIEW: type = "webview"; break;
      case Type::OFF_SCREEN: type = "offscreen"; break;
      default: break;
    }
    return mate::ConvertToV8(isolate, type);
  }
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,