theme_get_setting() tries to use a global variable which doesn't exist anymore

People keep trying to slip stuff referring to the global $custom_theme variable into Drupal even though that global variable no longer exists... probably it's related to the fact that we haven't gotten around to documenting the change from #553944: Define hook_menu_get_item_alter() as a reliable hook that runs before the page is doomed yet :)

In this case, I'm not even sure what the code was trying to use it for. Whenever drupal_theme_initialize() is called, the global $theme_key gets set to the actual theme that is going to be used for the current page, so as far as I can tell, that is all we would ever need to check here.