Although I have no idea what the core issue is (and probably, as stated by @undefinederror, this is just a symptom of a misconfiguration somewhere else, because the crontab area needing a theme makes little sense to me), I will add that in addition to the new record within the theme table, I also had to override the DI preferences for Magento\Theme\Model\...
If its is not working
Try deleting from the directory files:
And then run:
php bin/magento indexer:reindex
php bin/magento setup:di:compile
php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy -f
Then try to This.
Select luma theme on dashboard and then remove your theme from database.
Apart from above answers
I would like to add parent entities <page><body> for the default.xml file.
This is basically for the newcomers like me, because without this page and body our file will not reflected on the frontend.
here is my update default.xml file
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:...