10 files declare their use of ThemeHandlerInterface

ConfigEntityBaseUnitTest.php in core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityBaseUnitTest.php
Contains \Drupal\Tests\Core\Config\Entity\ConfigEntityBaseUnitTest.
ConfigImporter.php in core/lib/Drupal/Core/Config/ConfigImporter.php
ConfigImportSubscriber.php in core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php
FilesystemLoader.php in core/lib/Drupal/Core/Template/Loader/FilesystemLoader.php
LayoutPluginManager.php in core/lib/Drupal/Core/Layout/LayoutPluginManager.php
LayoutPluginManagerTest.php in core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php
PluginDependencyTraitTest.php in core/tests/Drupal/Tests/Core/Plugin/PluginDependencyTraitTest.php
Registry.php in core/lib/Drupal/Core/Theme/Registry.php
ThemeAccessCheck.php in core/lib/Drupal/Core/Theme/ThemeAccessCheck.php
ThemeInitialization.php in core/lib/Drupal/Core/Theme/ThemeInitialization.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.