Wordpress administration is gone after creating base theme

Question

I have following files for a base theme:

• wp-content/themes/base/style.css (empty)
• wp-content/themes/base/index.php

// index.php
<?php declare(strict_types=1);

echo wp_nav_menu();

After switching the theme to the base theme, http://localhost:8085 shows a base page with naviagtion items, so far so good. Now I want to add more navigation items in the admin area. But when I go to http://localhost:8085/wp-admin/, the same content is shown as in http://localhost:8085, wihtout administration toolbars.
How can I add pages or even change the theme back?

I can't believe a custom theme also affects the administration theme.