You are trying to change the theme in Content > Design: Configuration, but when you save the new setting, no matter which theme you choose, you get the following error:
Something went wrong while saving this configuration: Required parameter 'theme_dir' was not passed
As consequence, the new theme setting is not saved.
An easy solution would be to switch to a functioning theme by updating the setting in the database directly.
- Find out, which theme_id a stable theme has:
SELECT * FROM theme;
- Change the value for theme_id for the current store:
SELECT * FROM core_config_data WHERE path LIKE 'design/theme/theme_id'; UPDATE ...
- Flush configuration cache:
bin\magento cache:flush config