drupal_theme__admin/admin.theme

<?php
/**
 * admin: A very limited theme with almost no features, to maximize screen real estate
 *
 * @version $Id: admin.theme,v 1.1 2007-08-20 19:43:07 marand Exp $
 * @copyright 2006-2007 OSI
 * @license CeCILL 2.0
 */

function admin_settings()
  {
  $title = t('Extended settings for theme %theme', array('%theme' => 'admin'));
  drupal_set_title($title);
  $form = array();

  $form['admin_background'] = array
    (
    '#type'          => 'textfield',
    '#title'         => t('background-color'),
    '#default_value' => variable_get('admin_background', 'transparent'),
    '#description'   => t('CSS string for a background color. It will be applied to nodes.'),
    );

  return system_settings_form('admin_admin_settings', $form);
  }