no maintenance mode in theme

If so, it must have been a customized template or whatever for displaying some kind of “Maintenance” page while developing the theme. If you need something like that, a simple HTML page can can be used without actually disabling anything:
https://www.nnysandbox.net/maintenance.html
But if you want to completely hide your site and disable navigation, there are plugins that can do that:
https://www.ads-software.com/plugins/search.php?q=maintenance