How are new design themes registered on «bin/magento setup:upgrade»?

Step 1. Magento\Theme\Setup\RecurringData::install()

Step 2. Magento\Theme\Model\Theme\Registration::register()

Step 3A. (Magento 2.2) Magento\Framework\View\Design\Theme\ThemeList::loadData()

Step 3B. (Magento 2.1.6) Magento\Theme\Model\Theme\Collection::loadData()

Step 4. Magento\Framework\View\Design\Theme\ThemePackageList::getThemes()

Step 5. Magento\Theme\Model\Theme\Registration::_registerThemeRecursively()