I am not sure what is going on with your installation…
When I Open appearance on the dashboard, the URL I see is
https://SiteURL/folderInstall/wp-admin/themes.php
And so when I close the customizer it goes back to that URL.
Can you tell us what is the URL you see when you click on Appearance?
Note: You no longer need the “Easy theme and plugin upgrade” Plugin, as the functionality is now part of WordPress