Location of adminimal-custom.css

Adding a patch we have been using on Acquia

For what it's worth, I've placed the following in /sites/default/files/adminimal-custom.css:
@import "/themes/custom/my_theme/css/adminimal-custom.css";

This allows me to commit my changes to source control, without having to muss about in the files directory.

if you rebuild with a file in the directory, you get:

The website encountered an unexpected error. Try again later.

Drupal\Core\File\Exception\FileExistsException: File 'temporary://fileIb5W5B' could not be copied because a file by that name already exists in the destination directory (''). in Drupal\Core\File\FileSystem->prepareDestination() (line 464 of core/lib/Drupal/Core/File/FileSystem.php).
Drupal\Core\File\FileSystem->move('temporary://fileIb5W5B', , Object) (Line: 490)
Drupal\Core\File\FileSystem->saveData('', 'public://adminimal-custom.css', Object) (Line: 71)
adminimal_theme_form_system_theme_settings_alter(Array, Object) (Line: 384)
Drupal\system\Form\ThemeSettingsForm->buildForm(Array, Object, 'adminimal_theme')
call_user_func_array(Array, Array) (Line: 536)
Drupal\Core\Form\FormBuilder->retrieveForm('system_theme_settings', Object) (Line: 284)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 53)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 116)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 90)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 50)
Drupal\ban\BanMiddleware->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 741)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)