code: public Collection<Theme> getChildThemes(ThemeFilter filter) {
          filter = (filter != null) ? filter : new ThemeFilter();
  
          filter.parent.clear();
          filter.parent.add(this);
          return getInstance().get().themes(filter);
      }
docs: Child Themes of this Theme.
  
  @param filter Criteria to filter children of themes. Pass a ThemeFilter
  to get only children of themes, respectively.
  @return A collection children of theme that belong to this theme filtered
  by the passed in filter.