8 files declare their use of ThemeNegotiatorInterface in Zircon Profile 8

AdminDemoNegotiator.php in core/modules/block/src/Theme/AdminDemoNegotiator.php

Contains \Drupal\block\Theme\AdminDemoNegotiator.

AdminNegotiator.php in core/modules/user/src/Theme/AdminNegotiator.php

Contains \Drupal\user\Theme\AdminNegotiator.

BatchNegotiator.php in core/modules/system/src/Theme/BatchNegotiator.php

Contains \Drupal\system\Theme\BatchNegotiator.

CustomThemeNegotiator.php in core/modules/system/tests/modules/theme_test/src/Theme/CustomThemeNegotiator.php

Contains \Drupal\theme_test\Theme\CustomThemeNegotiator.

DbUpdateNegotiator.php in core/modules/system/src/Theme/DbUpdateNegotiator.php

Contains \Drupal\system\Theme\DbUpdateNegotiator.

HighPriorityThemeNegotiator.php in core/modules/system/tests/modules/theme_test/src/Theme/HighPriorityThemeNegotiator.php

Contains \Drupal\theme_test\Theme\HighPriorityThemeNegotiator.

MenuTestController.php in core/modules/system/tests/modules/menu_test/src/Controller/MenuTestController.php

Contains \Drupal\menu_test\Controller\MenuTestController.

TestThemeNegotiator.php in core/modules/system/tests/modules/menu_test/src/Theme/TestThemeNegotiator.php

Contains \Drupal\menu_test\Theme\TestThemeNegotiator.