Instead of:
alertDialog.getButton(DialogInterface.BUTTON_POSITIVE) .setBackground(getResources().getDrawable(R.drawable.btn_bar_holo_red));
Try:
alertDialog.setOnShowListener(new DialogInterface.OnShowListener() {
@Override
public void onShow(DialogInterface dialog) {
Button positiveButton = ((AlertDialog) dialog)
.getButton(AlertDialog.BUTTON_POSITIVE);
positiveButton.setBackgroundResource(R.drawable.btn_bar_holo_red_full);
Button negativeButton = ((AlertDialog) dialog)
.getButton(AlertDialog.BUTTON_NEGATIVE);
negativeButton.setBackgroundResource(R.drawable.btn_bar_holo_red_full);
}
});
Where btn_bar_holo_red_full would be defined in it's own xml file as follows:
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:drawable="@drawable/button_pressed" android:state_pressed="true" />
<item android:drawable="@drawable/button_focused" android:state_focused="true" />
<item android:drawable="@drawable/button_default" />
</selector>
Then define button_pressed, button_focused and button_default as follows (you'll need three, one each for pressed, focused and default):
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" androuid:shape="rectangle">
<solid android:color="@color/btn_background_pressed" />
</shape>
All that's left to do is to add those color values to your color.xml file.