XB's drupal.dialog needs to account for non-core admin theme overrides

This is most easily tested with a Gin-integrated setup like DrupalCMS, the change is fairly easy to paste in and might be the simplest way to manually review.

I trust you tested this thoroughly, I don't feel the need to double-check a well-commented MR 👏

Automatically closed - issue fixed for 2 weeks with no activity.