16 files declare their use of ThemeSettingsProvider

BareHtmlPageRenderer.php in core/lib/Drupal/Core/Render/BareHtmlPageRenderer.php
CommentThemeHooks.php in core/modules/comment/src/Hook/CommentThemeHooks.php
NodeThemeHooks.php in core/modules/node/src/Hook/NodeThemeHooks.php
olivero.theme in core/themes/olivero/olivero.theme
Functions to support theming in the Olivero theme.
OliveroPagePreprocessHooks.php in core/themes/olivero/src/Hook/OliveroPagePreprocessHooks.php
ShortcutThemeHooks.php in core/modules/shortcut/src/Hook/ShortcutThemeHooks.php
SystemBrandingBlock.php in core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
test_theme_theme.theme in core/modules/system/tests/themes/test_theme_theme/test_theme_theme.theme
theme-settings.php in core/modules/system/tests/themes/test_theme_settings/theme-settings.php
theme.api.php in core/lib/Drupal/Core/Render/theme.api.php
Hooks and documentation related to the theme and render system.
theme.inc in core/includes/theme.inc
The theme system, which controls the output of Drupal.
ThemeInstallerTest.php in core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php
ThemePreprocess.php in core/lib/Drupal/Core/Theme/ThemePreprocess.php
ThemeSettingsForm.php in core/modules/system/src/Form/ThemeSettingsForm.php
ThemeSettingsTest.php in core/tests/Drupal/KernelTests/Core/Theme/ThemeSettingsTest.php
UserThemeHooks.php in core/modules/user/src/Hook/UserThemeHooks.php

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