message: Fix theme loading in open-sourced-themes example.
  Related-to:
diff: import { GeoCoordinates } from "@here/harp-geoutils";
  import { MapControls, MapControlsUI } from "@here/harp-map-controls";
  -import { CopyrightElementHandler, MapView } from "@here/harp-mapview";
  +import { CopyrightElementHandler, MapView, ThemeLoader } from "@here/harp-mapview";
  import { APIFormat, OmvDataSource } from "@here/harp-omv-datasource";
  import { GUI } from "dat.gui";
  import { accessToken, copyrightInfo } from "../config";
  @@ -69,11 +69,7 @@ export namespace ThemesExample {
  };
  gui.add(options, "theme", options.theme)
  .onChange((value: string) => {
  - fetch(value)
  - .then(response => {
  - return response.json();
  - })
  - .then((theme: any) => {
  + ThemeLoader.load(value).then(theme => {
  mapView.theme = theme;
  });
  })
  