Magento 2 Required parameter 'theme_dir' was not passed

Magento 2 Required parameter 'theme_dir' was not passed

STEP 1 : File Operations

Delete the content from app/design/frontend/<Vendor> (make sure you back it up first).

Delete all the content of folder var/view_preprocessed & pub/static/frontend.

rm -rf var/view_preprocessed/* pub/static/frontend/*

STEP 2 : DB Operations

Go to your theme table and delete the entry for your created theme.

After that...

Go to your core_config_data table and search for theme and you will get path records like design/theme/theme_id replace your default theme id in it.

STEP 3 : Flush cache

Flush your cache php bin/magento cache:flush

Let me know if you still face any problems after that.

That's all.