In a custom subtheme of classy base theme, I made a theme setting for referencing an image media file. Contents of file theme-settings.php:

<?php use DrupalCoreFormFormStateInterface; use DrupalCoreThemeThemeSettings; use DrupalsystemFormThemeSettingsForm; use DrupalCoreForm; function mytheme_form_system_theme_settings_alter(&$form, DrupalCoreFormFormStateInterface $form_state) {  $opts = [ '#type'          => 'entity_autocomplete', '#title'         => t("Image on startpage"), '#description'   => t("Referencing an image."), '#target_type'   => 'media', '#selection_settings' => ['target_bundles' => ['image']], ]; if ($default_id = theme_get_setting('startpage_image')) { // element stores an int ID, but default value has to be the loaded entity $image = Drupal::entityTypeManager()->getStorage('media')->load($default_id); $opts['#default_value'] = $image; } $form['startpage_image'] = $opts; } ?> 

Contents of file mytheme.theme:

<?php  function mytheme_preprocess_page(&$vars) { $media_id = theme_get_setting('startpage_image'); $media = Drupal::entityTypeManager()->getStorage('media')->load($media_id); $variables['media_build'] = Drupal::entityTypeManager()->getViewBuilder('media')->view($media, 'media.full'); }  ?> 

And finally in the template page--front.html.twig the image is called by {{ startpage_image }}. But unfortunately, the image isn’t shown (all caches rebuilt and flushed multiple times). In the last line in file mytheme.theme I also tried view($media, 'full') because I wasn’t sure if media.full is the correct display mode that is built in into media core module — but this as well wasn’t successfull.

Why is my code not working?