How to add Specific Border to Labels

I am making a Sudoku Solver in a JavaFx Application. I have got the grid ready as shown below, but I don't know how to add the center border to distinguish between the smaller boxes.

picture of grid

Like in a normal sudoku, I want to get the border between the smaller boxes.

Here's is the code:-

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;
}