Add body class to both Node add and Node Edit Page
Did you try :

function MYTHEME_preprocess_html(&amp;$variables) {
  if ((arg(0)=='node' &amp;&amp; arg(2)=='edit') || (arg(0) == 'node') &amp;&amp; (arg(1) == 'add')) {
    $variables['classes_array'][] = 'node-edit-page';
  }
}