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