You can just set the path to /admin but what if there are other pages that you want to have the admin theme like /node/edit . . ..

I needed to do this recently so knocked together this quick module, this hooks into the  admin/build/themes/admin page. You will see 2 custom text areas, one for pages that you want to force to use your sites theme and one for pages that you want to use the admin theme. Works fine most pages but dont use it for the blocks page as this won't show your blocks.

Hope this helps

<?php

/**
 * Implementation of hook_init().
 */
function admin_theme_additions_init() {
  $admin_theme_disallow = FALSE;
  $admin_theme = FALSE;
 
  // check if some paths are disallow to get the theme
  if (trim(variable_get('admin_theme_additions_path_disallow', '')) != '') {
    // pages that are defined by their normal path
    $admin_theme_disallow = drupal_match_path($_GET['q'], variable_get('admin_theme_additions_path_disallow', ''));
   
    // pages that are defined with their alias
    $alias = drupal_get_path_alias($_GET['q']);
    if ($alias != $_GET['q']) {
      $admin_theme_disallow = $admin_theme || drupal_match_path($alias, variable_get('admin_theme_additions_path_disallow', ''));
    }
  }

  // we should not show the admin theme if the user has no access or the path is in the disallow list
  if ($admin_theme_disallow) {
    unset($GLOBALS['custom_theme']);
    return;
  }
   


  // some custom defined pages should get admin theme
  if (trim(variable_get('admin_theme_additions_path', '')) != '') {
 
    // pages that are defined by their normal path
    $admin_theme = $admin_theme || drupal_match_path($_GET['q'], variable_get('admin_theme_additions_path', ''));

    // pages that are defined with their alias
    $alias = drupal_get_path_alias($_GET['q']);
    if ($alias != $_GET['q']) {
      $admin_theme = $admin_theme || drupal_match_path($alias, variable_get('admin_theme_additions_path', ''));
    }
  }
 
  // trun the admin theme on or off depedning on the results above
  if ($admin_theme == TRUE){
    _admin_init_theme();
  }
}


function admin_theme_additions_form_system_admin_theme_settings_alter(&$form, $form_state) {

  // allow the user to define a set of pages where the admin theme should or should not be applied to
  $form['pages']['custom'] = array(
    '#type' => 'fieldset',
    '#title' => t('Custom pages'),
    '#description' => t("Enter one page per line as Drupal paths. The '*' character is a wildcard. Example paths are %blog for the blog page and %blog-wildcard for every personal blog. %front is the front page.", array('%blog' => 'blog', '%blog-wildcard' => 'blog/*', '%front' => '<front>')),
    '#collapsible' => TRUE,
    '#collapsed' => TRUE,
    '#weight' => 9,
  );
  $form['pages']['custom']['admin_theme_additions_path'] = array(
    '#type' => 'textarea',
    '#title' => t('Use administration theme on the following pages.'),
    '#default_value' => variable_get('admin_theme_additions_path', ''),
  );
  $form['pages']['custom']['admin_theme_additions_path_disallow'] = array(
    '#type' => 'textarea',
    '#title' => t('Do not use administration theme on the following pages.'),
    '#description' => t('If a path appears here, the administration theme is not shown even if all above options apply.'),
    '#default_value' => variable_get('admin_theme_additions_path_disallow', ''),
  );

}
?>