A mains powered zwave switch Will work just like a regular switch even if your home automation system is down. It has a radio inside which is what allows it to talk to your smartthings hub.
The battery powered zwave switches have a similar radio so they can communicate with the SmartThings, but they do not control electricity going to the light fitting. So if your home automation system is down, that switch won’t do anything.
That’s why most people using battery operated switches keep the original switch as well, so that they still have control over the lights if their Internet or home automation system is unavailable.
Here’s a very nice looking box cover project:
But again, if you have a three wire set up at the switchbox, you can just use a mains power switch and you don’t have to worry about a cover.