docs: public CSSClass getClass(String name, Object owner) throws CSSNamingConflict {
      CSSClass existing = store.get(name);
      // Not found.
      if (existing == null) {
        return null;
      }
      // Different owner
      if (owner != null && existing.getOwner() != owner) {
        throw new CSSNamingConflict("CSS class naming conflict between "+owner.toString()+" and "+existing.getOwner().toString());
      }
      return existing;
    }