Magento 2.x: Required parameter ‘theme_dir’ was not passed

This error message appears in Magento 2 when you deleted a theme directory and the theme registry reminds in the database. To fix this issue you need to go to app/design/frontend and check which themes you have installed. Then go to the database and execute the following command:

SELECT * FROM theme;

Check what is the extra theme in that table and remove the registry. Clear cache and that should be it.