The update was spotted by 9to5Google.com, who found it in the release notes for the iOS app. It hasn't been rolled out on iOS yet, though: it's due "over the next few weeks" according to Google, which is when we'd expect the Android version to drop too.
Google Home app redesign: do more things more quickly
The changes to the app make it look and work more like the Device controls page in version 11 onwards of Android, and according to Google it will "help you adjust your devices with fewer taps". That means faster access to your smart light controls, to the volume of music on your smart speaker, and easier access to more advanced controls by long-pressing the device icons. The background will also make it easier to see the status of a device, for example by making it obvious if your smart lights are on or off and how bright they are.
This might not seem like a huge update but if you're anything like me you're interacting with your smart home kit quite a lot, and all those little interactions add up – so anything that makes those interactions a little faster will make your smart home life a little better.
If you're on Android and you really can't wait for the update, you can get the update by sideloading the latest Google Home APK – it's version 18.104.22.168.