We have recorded that there have been some clients who were not experts at magento struggled when trying to uninstall theme following this article https://arrowhitech.ticksy.com/article/15889/
Normally we are not allowed to help any further at this point but we will explain a little bit
1 - Remove theme files: In your project source code, everywhere contains folder named "Mgs", please delete them all
2 - Edit "theme" table and remove the theme
When you installed theme, you already created tables automatically, they were named with prefix "mgs_", please delete them all as well
3 - In "eav_attribute" table, which is magento default table, please search for "source_model" then delete every elements with "mgs" like in the attachment
Deploy means "deploy command: which is php bin/magento setup:static-content:deploy