The following code should create a button with a mitered border similar to the inset border style seen here: http://www.w3schools.com/cssref/playit.asp?filename=playcss_border-style&preval=inset

button.setStyle("-fx-background-color: null; -fx-border-width: 5; -fx-border-color: black grey grey black; -fx-border-style: solid line-join miter 45;");

However the line-join does not work as expected as the joins end up square rather than mitered.

Refer to https://forums.oracle.com/forums/thread.jspa?threadID=2333665 for a related discussion.

Sample test program:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.*;
import javafx.stage.Stage;
public class MiteredButtonStyle extends Application {
  public static void main(String[] args) throws Exception { launch(args); }
  public void start(final Stage stage) throws Exception {
    Button button = new Button("My lines-joins are square, not mitered");
    button.setStyle("-fx-background-color: null; -fx-border-width: 5; -fx-border-color: black grey grey black; -fx-border-style: solid line-join miter 45;");
    stage.setScene(new Scene(VBoxBuilder.create().spacing(10).children(button).style("-fx-background-color: azure; -fx-padding: 20; -fx-font-size:20;").build()));
    stage.show();
  }
}