Android Studio AlertDialog Customize - Kotlin

MyDialogFragment.kt

class MyDialogFragment : DialogFragment() {

    override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
        /* 省略 */
    }

    override fun onStart() {
        super.onStart()
        (dialog as AlertDialog).getButton(AlertDialog.BUTTON_POSITIVE).setTextColor(Color.WHITE)
        (dialog as AlertDialog).getButton(AlertDialog.BUTTON_POSITIVE).setBackgroundColor(resources.getColor(R.color.orange))
    }
}