جعل الديالوغ شفاف

final AlertDialog dialog = new AlertDialog.Builder(MainActivity.this).create();
LayoutInflater inflater = getLayoutInflater();

View convertView = (View) inflater.inflate(R.layout.dialog, null);
dialog.setView(convertView);

dialog.getWindow().setBackgroundDrawableResource(android.R.color.transparent);

dialog.show();