code: func WithBaseFs(b *BaseFs) func(*BaseFs) error {
  	return func(bb *BaseFs) error {
  		bb.themeFs = b.themeFs
  		bb.AbsThemeDirs = b.AbsThemeDirs
  		return nil
  	}
  }
docs: // WithBaseFs allows reuse of some potentially expensive to create parts that remain
  // the same across sites/languages.