How can I customize nebular theme service variables?

Question

I'm currently working on an angular app that uses nebular/them pack, and I want to add some custom variables to the NbThemeService, I saw this customization in ngx-admin template as below:

  constructor(private ecMapService: CountryOrdersMapService,
          private theme: NbThemeService) {

combineLatest([
  this.ecMapService.loadCounties(null),
  this.theme.getJsTheme(),
])
  .pipe(takeWhile(() => this.alive))
  .subscribe(([cords, config]: [any, any]) => {
    this.currentTheme = config.variables.countryOrders;
    this.layers = [this.createGeoJsonLayer(cords)];
  });

here in this component constructor we have the countryOrders custom property on the variables object from this.getJsTheme(). How can I add my own custom properties to this theme service ?