docs: public CSSClass addClass(CSSClass clss) throws CSSNamingConflict {
      CSSClass existing = store.get(clss.getName());
      if (existing != null && existing.getOwner() != null && existing.getOwner() != clss.getOwner()) {
        throw new CSSNamingConflict("CSS class naming conflict between "+clss.getOwner().toString()+" and "+existing.getOwner().toString());
      }
      return store.put(clss.getName(), clss);
    }