Create a Style.css file:
-------------------------------------------------------
Circle { opacity: 0.5 }
-------------------------------------------------------

Create a JavaFX Script file:
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import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.shape.Circle;
import javafx.scene.paint.Color;


Stage {
    title: "Application title"
    width: 250
    height: 280
    scene: Scene {
        stylesheets: ["{__DIR__}Style.css"]
        content: [
            Circle {
                centerX: 100
                centerY: 100
                radius: 10
                stroke: Color.GREEN
            }

        ]
    }
}
-------------------------------------------------------

Run the File:

standard-run:
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.Float
        at com.sun.javafx.runtime.location.FloatVariable.set(FloatVariable.java:38)
        at javafx.reflect.FXLocal$VarMember.setValue(FXLocal.java:729)
        at com.sun.stylesheet.javafx.Attribute.setProperty(Attribute.java:153)
        at com.sun.stylesheet.javafx.FXObjectStyleSupport.setProperty(FXObjectStyleSupport.java:342)
        at com.sun.stylesheet.styleable.DefaultStyleable.setProperty(DefaultStyleable.java:168)
        at com.sun.stylesheet.PropertyManager.applyProperty(PropertyManager.java:594)
        at com.sun.stylesheet.PropertyManager.applyProperty(PropertyManager.java:555)
        at com.sun.stylesheet.Declaration.applyTo(Declaration.java:91)
        at com.sun.stylesheet.Rule.applyTo(Rule.java:221)
        at com.sun.stylesheet.Stylesheet.applyTo(Stylesheet.java:277)
        at com.sun.stylesheet.Stylesheet.applyTo(Stylesheet.java:282)
        at com.sun.stylesheet.Stylesheet.applyTo(Stylesheet.java:257)
        at javafx.scene.Scene.applyStylesheets$impl(Scene.fx:235)
        at javafx.scene.Scene.applyStylesheets(Scene.fx:43)
        at javafx.scene.Scene.userInit$(Scene.fx:220)
        at javafx.scene.Scene.initialize$(Scene.fx:43)
        at javafxapplication43.Main.javafx$run$(Main.fx:19)
        at javafxapplication43.Main.javafx$run$(Main.fx:19)