public MainWindow()
{
InitializeComponent();
repoUser = RepositoryUser.Instance;
repoTheme = RepositoryTheme.Instance;
repoAuthor = RepositoryAuthor.Instance;
repoBook = RepositoryBook.Instance;
repoUser.LoadData();
repoTheme.LoadData();
repoAuthor.LoadData();
repoBook.LoadData();
}