At me the following adjustments in a configuration file:
$emailadmin=1;
$emailusers=1;

To the admin there come letters on new users.
To users there come the letters notifying on new answers.
How to make so that to the admin there came letters on all new themes and answers?

Set $emailadmposts to 1. [Reference: Customization - config: 2nd step, miniBB Manual]. Read the Manual.
-mb

Thank you!!!

On a forum three moderators. How to make so that they too received such letters?

How to make so that they too received such letters
It's not available by default, but you may try to specify $admin_email to list all necessary emails there... like

$admin_email='email1@domain.com, email2@domain.com, email3@domain.com';

not sure it will work but you may try...