code: public Collection<Theme> getThemes(ThemeFilter filter) {
          filter = (filter != null) ? filter : new ThemeFilter();
  
          filter.goals.clear();
          filter.goals.add(this);
          return getInstance().get().themes(filter);
      }
docs: Themes assigned to this Goal.
  
  @param filter Limit the theme returned (If null, then all items
  returned).
  @return A collection themes that belong to this goal filtered by the
  passed in filter.