In Dialog.java (Android src) a ContextThemeWrapper is used. So you could copy the idea and do something like:
AlertDialog.Builder builder = new AlertDialog.Builder(new ContextThemeWrapper(this, R.style.AlertDialogCustom));
And then style it like you want:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="AlertDialogCustom" parent="@android:style/Theme.Dialog">
<item name="android:textColor">#00FF00</item>
<item name="android:typeface">monospace</item>
<item name="android:textSize">10sp</item>
</style>
</resources>