The feature is hidden behind a flag, but it's coming
Chrome OS has come a long way in the couple of years it's been around. With the big Aura update a year ago, Chrome OS looks and feels like a proper operating system and it's getting better all the time.But some things are still missing, even big, obvious things like the ability to add screensavers.
Google is now making it possible for developers to add Chrome OS screensavers to the Chrome Web Store for anyone to grab.
The feature isn't ready yet and there aren't any screensavers in the Web Store yet. But if you're running a recent version of Chrome OS, all you have to do is turn on the "Enable screensaver extensions" flag in chrome://flags.
There's only one screensaver at the moment, the old one from the first Cr-48 devices. The first implementation of the screensaver extension API only allows one screensaver at a time, installing a new one will remove your current screensaver, which turns on after two minutes in idle.