如何在标签上添加特定边框

Question

我正在用JavaFx应用程序制作Sudoku解算器。我已经准备好网格，如下所示，但是我不知道如何添加中心边框来区分较小的框。

就像普通的数独游戏一样，我想在较小的盒子之间找到边界。

这是代码：-

private GridPane getGridPane() {
    GridPane gridPane = new GridPane();

    gridPane.setGridLinesVisible(true);
    gridPane.setPadding(new Insets(10));
    gridPane.setHgap(0);
    gridPane.setHgap(0);

    for (int i = 0; i < 9; i++)
        for (int j = 0; j < 9; j++) {
            buttons[i][j] = new Label("");
            buttons[i][j].setStyle("-fx-font-weight: bold; -fx-font-size: 40");
            buttons[i][j].setAlignment(Pos.CENTER);
            buttons[i][j].setTextAlignment(TextAlignment.CENTER);
            buttons[i][j].setMaxSize(Double.MAX_VALUE, Double.MAX_VALUE);
        }

    ...

    for (int i = 0; i < 9; i++) for (int j = 0; j < 9; j++) GridPane.setConstraints(buttons[i][j], j, i);

    for (int i = 0; i < 9; i++)
        for (int j = 0; j < 9; j++) gridPane.getChildren().add(buttons[i][j]);

    GridPane.setHalignment(gridPane, HPos.CENTER);
    GridPane.setValignment(gridPane, VPos.CENTER);
    gridPane.setAlignment(Pos.CENTER);

    return gridPane;
}