[SalesForce] How to change live agent offline message

I ran into this problem today. The explanation is in the following article:

Live Agent Chat Buttons with Multiple Skills

No, your users will not need to log in. You can surface this functionality without requiring authentication by adding the Visualforce Page(s) in question to a Force.com Site (emphasis added):

Force.com Sites lets you create public web applications and web sites that run natively on Force.com - under your own domain. Force.com Sites lets you easily extend the reach of your existing applications to new, external audiences, and your site visitors don't need to authenticate to view your pages. In fact, if you're already using Visualforce to create your user interfaces, it's a simple matter of bundling them into a Force.com Site to create a public web application.