I started a new theme based on Magento/blank. Changes in the frontend area work as expected, but I want to change a <container> definition of Magento_Theme/view/base/page_layout/empty.xml . Since that lays outside of the frontend folder, magento seems to ignore my trys to overwrite/extend the file.
I already added the following files, according to the frontendversions, but adpated for base. All in this path: app/design/base/Vendor/Ext :
composer.json, registration.php, theme.xml and media/preview.jpg