Форуми / Cotonti / Extensions / Index Login

You can change the index.tpl

Or you put your site in the maintenance mode, and give ALL users access. You can also give a message in the box.
At that moment you only see the login. And only users can do something.

admin.php?m=config&n=edit&o=core&p=main

Maintenance mode :  Yes  No  Let only authorized personel access to site [R] 
Maintenance reason :   Optional, should better be short 

Change rights of all the users-groups:
admin.php?m=users

Access to site when maintenance mode on : Yes No