Error after Migration: Unable to load theme by specified key (Magento 2)
Thanks for your help.
I found 'themename' in database table design_change and delete the table content.
To fix this I used:
SELECT * FROM `core_config_data` WHERE `path` LIKE '%theme_id%'
and then deleted my custom theme records.
I also had to empty the Magento var/cache
directory.