In FXML:

<ComboBox fx:id="operationSelector" styleClass="singleton-combo-box" GridPane.columnIndex="1" GridPane.rowIndex="1" />

In CSS:

.singleton-combo-box:disabled {
  -fx-opacity: 1.0;
}
.singleton-combo-box .list-cell:disabled {
  -fx-opacity: 1.0;
  -fx-text-fill: black;
}

In Java:

	@FXML private ComboBox<String> operationSelector;

	@FXML
	private void initialize()
	{
		this.operationSelector.setDisable(true);
	}