customize check box preference

I am unable to customize my checkbox , although I have defined the background in the xml preference file, it doesn't pull the file. 1. I am trying to display custom images for checkbox and have defined the selector xml as "android_button.xml" which looks like :

   <?xml version="1.0" encoding="utf-8"?>
  <selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_checkable="true"
      android:drawable="@drawable/state_normal" /> <!-- pressed -->
<item  android:state_checked="true"
      android:drawable="@drawable/android_pressed" /> <!-- focused -->
<item android:drawable="@drawable/state_normal" /> <!-- default -->
</selector>

state_normal and android_pressed are two .png images in res>drawable folder.

2.my Checkbox preference.xml file is :

          <CheckBoxPreference android:key="@string/Drop_Option"
            android:title="Close after call drop"
            android:defaultValue="true"
            android:background="@drawable/android_button"
            />

Is there any error in the definition, The only change that shows up in screen is the android:title text, if I change the text, it changes the text. Nothing else changes. How do I fix this. Thank you for your suggestion.