custom_alert_dialog.kt

// Customizing title of Alertdilaog.
val builder = AlertDialog.Builder(this, R.style.CustomAlertDialog)
val tvTitle= TextView(this)
tvTitle.text = "Title of Alert Dialog!"
tvTitle.textSize = this.resources.getDimension(R.dimen.snack_bar_text_size)
builder.setCustomTitle(tvTitle)

// Cuatom default button
<style name="CustomAlertDialog" parent="@android:style/Theme.Dialog">   //  for white : Theme.AppCompat.Light.Dialog.Alert
        <item name="android:textSize">@dimen/snack_bar_text_size</item>
    </style>
// try this too
 <item name="android:bottomBright">@color/white</item>
        <item name="android:bottomDark">@color/white</item>
        <item name="android:bottomMedium">@color/white</item>
        <item name="android:textSize">10</item>
        <item name="android:centerBright">@color/white</item>
        <item name="android:centerDark">@color/white</item>
        <item name="android:centerMedium">@color/white</item>
        <item name="android:fullBright">@color/orange</item>
        <item name="android:fullDark">@color/orange</item>
        <item name="android:topBright">@color/blue</item>
        <item name="android:topDark">@color/blue</item>

// message
AlertDialog dialog = new AlertDialog.Builder(this).setMessage("Hello world").show();
    TextView textView = (TextView) dialog.findViewById(android.R.id.message);
    textView.setTextSize(40);