Base tpl file for "add content" page?
Try 

theme_node_add_list


Parameters

$variables: An associative array containing:

content: An array of content types.

function theme_node_add_list($variables) {
  $content = $variables['content'];
  $output = '';

  if ($content) {
    $output = '&lt;dl class="node-type-list"&gt;';
    foreach ($content as $item) {
      $output .= '&lt;dt&gt;' . l($item['title'], $item['href'], $item['localized_options']) . '&lt;/dt&gt;';
      $output .= '&lt;dd&gt;' . filter_xss_admin($item['description']) . '&lt;/dd&gt;';
    }
    $output .= '&lt;/dl&gt;';
  }
  else {
    $output = '&lt;p&gt;' . t('You have not created any content types yet. Go to the &lt;a href="@create-content"&gt;content type creation page&lt;/a&gt; to add a new content type.', array('@create-content' =&gt; url('admin/structure/types/add'))) . '&lt;/p&gt;';
  }
  return $output;
}


See here