fte_event_styles.module

<?php

function fte_event_styles_preprocess_html(&$variables) {

  global $theme;

  if ($theme === variable_get('admin_theme', 'seven')) {

    // Reference your custom stylesheet.
    drupal_add_css(drupal_get_path('module', 'fte_event_styles') . '/css/fte_event_styles.css', array('weight' => CSS_THEME));
  }
}

?>