drupal 7 admin theme only for certain roles
This is an annoyingly silly problem... with an annoyingly simple answer that took a bunch of time to find.
You want the admin theme to only show up for certain roles. There's a PERMISSION to allow for this.
Unfortunately, that answer doesn't come up when you search for it, and a bunch of available modules and hooks show up instead, none of which are useful. So I hope this helps you avoid the wasted time I spent on this.