Trying to add CSS to certain pages using a custom module in 7.x
To add CSS to node edit pages you can use hook_form_alter. Then check the $form_id to localize changes to desired node form(s) and add your call to drupal_add_css().

Example:

function admincss_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == "xxx") {
    drupal_add_css(drupal_get_path('module', 'admincss') . '/css/admincss.css');
  }
}


hook_preprocess_html() Is used to preprocess variables for the html.tpl.php file.