Base tpl file for "add content" page? I'm building my first drupal site, and I'm aware there's a huge amount about theming that I don't understand. Right now I'm learning to create custom layouts for specific pages by copying and modifying the template file for that page type (e.g., node.tpl.php -> node-—my-content-type.tpl.php). Now I'm trying to modify the "add content" page for a specific content type. Is there a template file that I can copy and modify, and if so, where can I find it? Or does this page type need to be handled differently?

This is in Drupal 7.

What I've tried:


Modifying node.tpl.php doesn't affect the add content page, nor does creating and modifying node--add.tpl.php nor node--add--my-content-type.tpl.php. 
Modifying page.tpl.php does affect the add content page (and all other pages). However...
Copying it to page--node--add--my-content-type.tpl.php (as suggested in this question) causes the add content page for my-content-type to not display at all, so apparently there is some other template info that's needed.
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