Use admin theme permission

If you want a new permission called "use admin theme" you can patch the module with these simple modifications:

Step 1: Create a new permission

Modify the admin_perm() function to this:

function admin_perm() {
  return array('use admin toolbar', 'use admin theme');
}

Step 2: Check permission

Add this code to admin_init():

// Use the administrative theme if the user is looking at a page in the admin/* path.
  if (!user_access('use admin theme')) {
    global $custom_theme;
    $custom_theme = variable_get('theme_default','none');;
  }

so that the function looks like this:

/**
 * Implementation of hook_init().
 */
function admin_init() {
  if (arg(0) === 'admin') {
    menu_set_active_menu_name('admin');
  }
  if (user_access('use admin toolbar')) {
    $path = drupal_get_path('module', 'admin');
    drupal_add_js("{$path}/includes/jquery.cookie.js");
    drupal_add_js("{$path}/includes/jquery.drilldown.js");
    drupal_add_js("{$path}/includes/admin.toolbar.js");
    drupal_add_js("{$path}/includes/admin.menu.js");
    drupal_add_css("{$path}/includes/admin.toolbar.base.css");
    drupal_add_css("{$path}/includes/admin.toolbar.css");
    drupal_add_css("{$path}/includes/admin.menu.css");
  }
  
  // Use the administrative theme if the user is looking at a page in the admin/* path.
  if (!user_access('use admin theme')) {
    global $custom_theme;
    $custom_theme = variable_get('theme_default','none');;
  }
}