Set AlertDialog Positive Button Text to be Bold

Here is my code.

AlertDialog.Builder builder = new AlertDialog.Builder(this);
    LayoutInflater inflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    final View vi = inflater.inflate(R.layout.group_dialog_layout,null);
    builder.setView(vi);
    TextView txtNewGroupEntry = (TextView) vi.findViewById(R.id.txtGroupRename);
    if(isNew==true){
        builder.setTitle("New Group");
        txtNewGroupEntry.setText(R.string.new_group_instruction);
    }
    builder.setPositiveButton(R.string.ok_button, null);
    builder.setNegativeButton(R.string.cancel_button, null);
    AlertDialog dialog = builder.create();
    dialog.show();
    Button okButton = dialog.getButton(DialogInterface.BUTTON_POSITIVE);

I have an alert dialog with an add button and a cancel button. I want both of the button's text to be bold and italic. How can I do it?