Add theme class to body and fix for theme selection (#16929)

apps/accessibility/appinfo/info.xml

apps/accessibility/composer/composer/autoload_classmap.php

apps/accessibility/composer/composer/autoload_static.php

apps/accessibility/css/dark.scss

apps/accessibility/css/highcontrast.scss

apps/accessibility/css/highcontrastdark.scss

apps/accessibility/img/mode-highcontrast.jpg

apps/accessibility/js/accessibility.js

apps/accessibility/js/accessibility.js.map

apps/accessibility/lib/AccessibilityProvider.php

apps/accessibility/lib/AppInfo/Application.php

.../accessibility/lib/Controller/AccessibilityController.php

apps/accessibility/lib/Controller/ConfigController.php

apps/accessibility/lib/Migration/RepairUserConfig.php

apps/accessibility/lib/Settings/Personal.php

apps/accessibility/src/App.vue

apps/accessibility/src/components/itemPreview.vue