Looked-up color fails for -fx-background-color in JavaFX CSS file

A DESCRIPTION OF THE PROBLEM :
If I change the stylesheet for a button, and the button's -fx-background-color references a looked-up color, JavaFX writes a message like this to the console.

Jun 08, 2021 2:38:28 PM javafx.scene.CssStyleHelper calculateValue
WARNING: Caught 'java.lang.ClassCastException: class java.lang.String cannot be cast to class javafx.scene.paint.Paint (java.lang.String is in module java.base of loader 'bootstrap'; javafx.scene.paint.Paint is in module javafx.graphics@11.0.11 of loader 'platform')' while converting value for '-fx-background-color' from rule '*.button-primary' in stylesheet file:/Users/will/github/CssTest/out/production/CssTest/csstest/dark.css


REGRESSION : Last worked in version 8u291

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
See the Java code and CSS files included below. Run the program. It displays a button labeled "CSS File". Press the button. The button will turn red, and the message will appear on the console.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Not to get an alarming message on the console.
ACTUAL -
Here is the message.

Jun 08, 2021 2:38:28 PM javafx.scene.CssStyleHelper calculateValue
WARNING: Caught 'java.lang.ClassCastException: class java.lang.String cannot be cast to class javafx.scene.paint.Paint (java.lang.String is in module java.base of loader 'bootstrap'; javafx.scene.paint.Paint is in module javafx.graphics@11.0.11 of loader 'platform')' while converting value for '-fx-background-color' from rule '*.button-primary' in stylesheet file:/Users/will/github/CssTest/out/production/CssTest/csstest/dark.css


---------- BEGIN SOURCE ----------
File: CssTest.java

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package csstest;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;


public class CssTest extends Application {
    static final String DARK =
        CssTest.class.getResource("dark.css").toExternalForm();
    static final String MEDIUM =
            CssTest.class.getResource("medium.css").toExternalForm();

    StackPane root;

    @Override
    public void start(Stage primaryStage) {
        Button btn = new Button();
        btn.setText("Set CSS");
        btn.setOnAction(evt -> {
            setTheme(MEDIUM);
        });
        btn.getStyleClass().clear();
        btn.getStyleClass().add("button-primary");
        
        root = new StackPane();
        root.getChildren().add(btn);
        root.getStyleClass().add("root");

        Scene scene = new Scene(root, 300, 250);
        
        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();

        setTheme(DARK);

    }

    private void setTheme(String theme) {
        System.out.println("setTheme: " + theme);
        // If I clear() the stylesheet, I get the messages.
        root.getStylesheets().clear();
        root.getStylesheets().add(theme);

        // If I add the new style sheet, and then remove any predecessors, I don't.
// while (root.getStylesheets().size() > 1) {
// root.getStylesheets().remove(0);
// }
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }
}

Plus two CSS files.

/* File: medium.css */
.button-primary {
-fx-background-color: -theme-button;
    -fx-font-family: helvetica;
}
.root {
-theme-button: #FF0000;
}

/* File: dark.css */
.button-primary {
-fx-background-color: -theme-button;
    -fx-font-family: helvetica;
}
.root {
-theme-button: #0679DE;
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
The problem doesn't occur if the java code adds the new stylesheet and then removes the old one instead of calling clear() on getStylesheets(); see the code.

In addition, if I remove the -fx-font-family style from button-primary, the message does not appear.

FREQUENCY : always