The textFill var cannot be styled on CheckBoxSkin although it works fine on ButtonSkin and RadioButtonSkin.

Test code:

Stage {
    title: "Application title"
    width: 250
    height: 80
    scene: Scene {
        stylesheets: "{__DIR__}checkstyle.css"
        content: [
          CheckBox {
            text: "Should Be Green"
          }
        ]
    }
}


/* checkstyle.css */

CheckBox {
  width: 200;
  fill: green;
  textFill: green;
  font: bold 18pt times;
}