Pane2.java

package sample;

import javafx.application.Application;
import javafx.collections.ObservableList;
import javafx.geometry.Insets;

import javafx.geometry.Pos;
import javafx.scene.layout.*;
import javafx.scene.control.*;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Pane2 extends Application {
    
    public static void main(String[] args) {
        launch(args);
    }

    public void start(Stage primaryStage) throws Exception {

        Tab predefinedTab = new Tab("new Tab");
        TabPane tabs = new TabPane(predefinedTab);

        //just add 1 more button. No round corners
        Button normalAddTab = new Button("Add tab");
        normalAddTab.setOnMouseClicked(event -> tabs.getTabs().addAll(new Tab("Tab " + (tabs.getTabs().size() + 1))));

        //this will update all tabs
        Button button2 = new Button("Update Style");
        button2.setOnMouseClicked(event -> {
            tabs.setId(null);
            tabs.setId("my-tab-pane");
        });

        //this will add 1 more tab and update all tabs except the last one
        Button button3 = new Button("Add and update tabs");
        button3.setOnMouseClicked(event -> {
            ObservableList tabsList = tabs.getTabs();
            Tab tab = new Tab("Tab " + (tabs.getTabs().size() + 1));
            tabsList.addAll(tab);
            tabs.setId(null);
            tabs.setId("my-tab-pane");
        });

        //same as button3
        Button button4 = new Button("Add styled tabs");
        button4.setOnMouseClicked(event -> {
            Tab tab = new Tab("Tab " + (tabs.getTabs().size() + 1));
            tabs.getTabs().add(tab);
            tabs.getStyleClass().remove("tab-pane");
            tabs.getStyleClass().add("tab-pane");
        });

        VBox root = new VBox();
        root.setAlignment(Pos.CENTER);
        root.setSpacing(10);
        root.setPadding(new Insets(10,10,10,10));
        root.setPrefSize(700,700);
        root.getChildren().addAll(tabs, normalAddTab, button2, button3, button4);

        Scene scene = new Scene(root);

        String css = Pane2.class.getResource("styles.css").toExternalForm();
        System.out.println(css);
        scene.getStylesheets().clear();
        scene.getStylesheets().add(css);

        primaryStage.setScene(scene);
        primaryStage.show();
    }
}

styles.css

.tab:selected .focus-indicator {
    -fx-border-radius: 10 10 0 0, 10 10 0 0;
    -fx-border-insets: -7 -7 -9 -8, -5 -5 -9 -6;
}

.tab-pane > .tab-header-area > .headers-region > .tab:selected{
    -fx-border-insets: 10 10 10 10, 10 10 10 10;
}

.tab-pane > .tab-header-area > .headers-region > .tab > .tab-container >.tab-label {
    -fx-alignment: CENTER;
    -fx-text-fill: -fx-text-base-color;
    -fx-padding:0 10 0 0;
}

.tab-header-area .tab{
    -fx-padding:4 10 5 10;
    -fx-border-radius: 10 10 0 0;
    -fx-background-radius: 10 10 0 0;
}