Issue #3457863 by grimreaper, catch, pdureau, maboy, longwave: YAML discovery...

      $this->list = [];

      $installed_themes = $this->configFactory->get('core.extension')->get('theme');

      if (!empty($installed_themes)) {

        $installed_themes = array_intersect_key($this->themeList->getList(), $installed_themes);

        array_map([$this, 'addTheme'], $installed_themes);

        $list = $this->themeList->getList();

        foreach (array_keys($installed_themes) as $theme_name) {

          $this->addTheme($list[$theme_name]);

        }

      }

    }

    return $this->list;

name: 'Theme test child theme'

type: theme

version: VERSION

base theme: test_parent_theme

theme_parent_provided_layout:

  class: '\Drupal\Core\Layout\LayoutDefault'

  label: Child

name: 'Theme test parent theme'

type: theme

version: VERSION

base theme: false

theme_parent_provided_layout:

  class: '\Drupal\Core\Layout\LayoutDefault'

  label: Parent