Uninstall or delete magento2 custom theme manually
How to uninstall or delete magento2 custom theme manually?
we can uninstall a custom theme with the following steps
1. Remove the theme folder from app/design/frontend/Vendor/
2. Clear the content of var/view_preprocessed/
3. Clear the content of pub/static/frontend/
4. Open the Magento’s database, go to the theme table then search your theme (which you are going to delete)and note the value of “theme_id”. Then delete the row.
5. In the table ‘core_config_data’ find the theme_id using below query.
SELECT * FROM `core_config_data` WHERE `path` LIKE '%theme_id%'
Check the value. If it contains the deleted theme_id value, then delete that row also.
6. Clear the cache with php bin/magento cache:flush
For more details please refer the below link