I’ll forgo the PROBLEM/SOLUTION in this one since it’s so quick.
I wanted to make the pages (forms) of a custom module I was creating use the admin theme instead of the public facing theme. Turns out it was really easy.
In the .routing.yml file, add the following:
options: _admin_route: TRUE
So a cleaned-up example of the yml looks like this:
whatever_your_form_is.form: path: '/whatever/your/path/is' defaults: _form: '\Drupal\module_name\Form\WhateverYourFormIs' _title: 'Page/Form Title' requirements: _role: 'authenticated' options: _admin_route: TRUE
Need this in my arsenal.