The Parent Theme Lock feature will prevent the user from deleting the Parent theme of an active Child theme.
This feature will help avoid accidental deletions and breaking a Child site’s look.
If you wish to delete a theme attached to an active Child theme, you first need to activate a different theme.
There are two places where the Parent Theme Lock is in effect on your Dashboard.
On your Manage Theme Page and in the Themes Widget in Individual Child Site mode.