Main.java

import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextField;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

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

    private static void swap(int[] array, int one, int two) {
        int temp = array[one];
        array[one] = array[two];
        array[two] = temp;
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        VBox root = new VBox();
        final TextField textField = new TextField();

        textField.getStyleClass().add("textfieldstyle");

        Button buttonRemoveAll = new Button("Reset");
        buttonRemoveAll.setOnMouseClicked(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent event) {
                textField.getStyleClass().clear();
                textField.getStyleClass().add("text-field");
                textField.getStyleClass().add("text-input");

                System.out.println(textField);
            }
        });

        Button buttonRemoveStyle1 = new Button("Remove style 1");
        buttonRemoveStyle1.setOnMouseClicked(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent event) {
                textField.getStyleClass().remove("textfieldstyle");
                System.out.println(textField);
            }
        });

        Button buttonAddStyle1 = new Button("Add style 1 (red)");
        buttonAddStyle1.setOnMouseClicked(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent event) {
                textField.getStyleClass().add("textfieldstyle");
                System.out.println(textField);
            }
        });

        Button buttonRemoveStyle2 = new Button("Remove style 2");
        buttonRemoveStyle2.setOnMouseClicked(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent event) {
                textField.getStyleClass().remove("textfieldstyle");
                System.out.println(textField);
            }
        });
        Button buttonAddStyle2 = new Button("Add style 2 (green)");
        buttonAddStyle2.setOnMouseClicked(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent event) {
                textField.getStyleClass().add("textfieldstyle2");
                System.out.println(textField);
            }
        });

        root.getChildren().addAll(textField, buttonAddStyle1, buttonRemoveStyle1 ,buttonAddStyle2,buttonRemoveStyle2, buttonRemoveAll);

        Scene scene = new Scene(root, 300,300);
        String css = Main.class.getResource("/styles.css").toExternalForm();
        scene.getStylesheets().clear();
        scene.getStylesheets().add(css);


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

styles.css

.textfieldstyle{
    -fx-background-color: red;
}
.textfieldstyle2{
    -fx-background-color: green;
}