Development Mode

How can I set Grav to Dev-Mode?

Is there a setting to make the site private?

Really the best option is to simply use .htaccess or something similar to ensure a user is logged in for development purposes. Alternatively you could look at the maintenance plugin.

thank you, but could you explain the first option in a bit more depth?

Lots of resources on this on the net as it's not a Grav-specific thing: http://www.htaccesstools.com/articles/password-protection/