Is it possible to theme node add form in drupal 7
The $variables['theme_hook_suggestions'][] only suggests theme for the node itself not the form to add content!

You have to let us know how exactly do you want to display your form. Some times a few CSS codes may be enough.

Anyway to theme the node add content you can use hook_form_alter to add some attributes to the form elements, add CSS classes or ... . You can also use Firebug to get the fields ID or class names then use CSS to theme them.

The following code 

function THEME_form_alter(&amp;$form, $form_state, $form_id) { 
      print $form_id;
}


gives you the ID of the form. Find the ID of the node add form and do the changes. like

function THEME_form_alter(&amp;$form, $form_state, $form_id) { 
      if($form_id == "NODE_ADD_FORM")
            ////
}