Dynamically change JavaFX css property

The method getStyleClass() returns a list of CSS class names associated with the node. These class names are used to determine which rules in an external CSS file are applied to the node. You cannot pass CSS selectors and rules in here.

Instead, write an external CSS file which contains a rule for the background color for the track. You can use a "looked-up color" here, which basically works like a "CSS variable". Define the "looked-up color" for the slider, and use it in a rule for the track to set the background color:

style.css:

.slider {
    -track-color: white ;
}
.slider .track {
    -fx-background-color: -track-color ;
}

Now, in the Java code, you can update the value for the looked-up color by calling setStyle("-track-color: /* someColor */") on the slider. Here is a quick example:

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.Slider;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class SliderTest extends Application {

    @Override
    public void start(Stage primaryStage) {
        Slider slider = new Slider();
        slider.valueProperty().addListener((obs, oldValue, newValue) -> {
            double percentage = 100.0 * (newValue.doubleValue() - slider.getMin()) / (slider.getMax() - slider.getMin());
            slider.setStyle("-track-color: linear-gradient(to right, #90C7E0 " + percentage+"%, white "+percentage+("%);"));
        });
        StackPane root = new StackPane(slider);
        root.setPadding(new Insets(20));
        Scene scene = new Scene(root);
        scene.getStylesheets().add("style.css");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

In your application, you can do the same using the current time of the media player (or just register a listener on the slider's value property, as that is changing too).