* 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
WindowOpenDisposition val) {
    std::string disposition = "other";
    switch (val) {
      case WindowOpenDisposition::CURRENT_TAB:
        disposition = "default";
        break;
      case WindowOpenDisposition::NEW_FOREGROUND_TAB:
        disposition = "foreground-tab";
        break;
      case WindowOpenDisposition::NEW_BACKGROUND_TAB:
        disposition = "background-tab";
        break;
      case WindowOpenDisposition::NEW_POPUP:
      case WindowOpenDisposition::NEW_WINDOW:
        disposition = "new-window";
        break;
      case WindowOpenDisposition::SAVE_TO_DISK:
        disposition = "save-to-disk";
        break;
      default:
        break;
    }
    return mate::ConvertToV8(isolate, disposition);
  }
};
template<>
struct Converter<content::SavePageType> {
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,