Note: this list is generated by looking for the string for this theme hook, so it may include some references that are not actually using this theme hook.

AJAXFrameworkTestCase::testLazyLoadOverriddenCSS in core/modules/simpletest/tests/ajax.test
Tests that overridden CSS files are not added during lazy load.
ajax_test_system_theme_info in core/modules/simpletest/tests/ajax_test.module
Implements hook_system_theme_info().
IconTestCase::testThemeIcons in core/modules/simpletest/tests/icon.test
Tests a theme providing and overriding icons, including sub-themes.
SystemThemeFunctionalTest::testPerThemeSettings in core/modules/system/tests/system.test
Test the individual per-theme settings form.
theme-settings.php in core/modules/simpletest/tests/themes/test_theme/theme-settings.php
ThemeDebugMarkupTestCase::setUp in core/modules/simpletest/tests/theme.test
Sets up a Backdrop site for running functional and integration tests.
ThemeDebugMarkupTestCase::testDebugNodeOutput in core/modules/simpletest/tests/theme.test
Tests debug node markup added to template output.
ThemeDebugMarkupTestCase::testDebugTaxonomyOutput in core/modules/simpletest/tests/theme.test
Tests debug taxonomy markup added to template output.
ThemeDebugMarkupTestCase::testDebugUserOutput in core/modules/simpletest/tests/theme.test
Tests debug user markup added to template output.
ThemeUnitTest::setUp in core/modules/simpletest/tests/theme.test
Sets up a Backdrop site for running functional and integration tests.
ThemeUnitTest::testListThemes in core/modules/simpletest/tests/theme.test
Test the list_themes() function.
ThemeUnitTest::testTemplateOverride in core/modules/simpletest/tests/theme.test
Ensures a themes template is overrideable based on the 'template' filename.
ThemeUnitTest::testThemeGetSetting in core/modules/simpletest/tests/theme.test
Test the theme_get_setting() function.
theme_test_system_theme_info in core/modules/simpletest/tests/theme_test.module
Implements hook_system_theme_info().
_theme_custom_theme in core/modules/simpletest/tests/theme_test.module
Custom theme callback.