How do I prevent D7's ajax_deliver() from loading CSS?
Figured out the reason Bartik CSS was loading when the admin page was using Seven.

My admin page was located at /admin/mymodule/settings

My menu callback for the AJAX link was /mymodule/special-form

Because /mymodule is outside the admin area, the theme would be Bartik when actually visiting that page. So Drupal's AJAX system assumes I want the CSS that goes along with that page. Not a bug, but a feature to be aware of.