alertDialog..배경..

Question

builder = new AlertDialog.Builder(con)
     .setTitle("선택하세요.")
     .setSingleChoiceItems(item, 0, new DialogInterface.OnClickListener() {
      
      @Override
      public void onClick(DialogInterface dialog, int which) {
       // TODO Auto-generated method stub
       
      }
     })
     .setPositiveButton("OK", new DialogInterface.OnClickListener() {
      
      @Override
      public void onClick(DialogInterface dialog, int which) {
       // TODO Auto-generated method stub
       
      }
     });
  alert = builder.create();
  alert.getWindow().setBackgroundDrawableResource(R.drawable.input_dark);
  alert.show();

아무리해도.. 배경이 안바뀌네요 ㅜㅜ

alert.getWindow().setBackgroundDrawableResource(R.drawable.input_dark);

고수님들 조언좀 부탁드립니다;; 이거 혹시 커스텀해야되는건가요?;;;;