Nova Launcher is one of the more popular third-party launchers out there. It's a free launcher, although there is a paid version which will get you a few more features for Nova Launcher. This launcher is quite popular because it brings an AOSP look and feel to just about any Android device, and it updates quite often with new features and UI changes, as is the case with this update. Nova Launcher does have a beta track, which uses the Google Play beta system (you'll have to join the Google+ Community and then opt into the beta), those features that debut in the beta version later come to the stable version, and that's the case today with the update to version 4.3.
Google announced the Android N Developer Preview back in March, which included some new folder icons. Which Nova added to their launcher in April. It was in the beta version of Nova Launcher, but as of today it has made its way into the stable version. On top of that, we also have Night Mode available in this update. Night Mode is just what you would expect. It can theme certain parts of the launcher to be black or a very dark gray, instead of the typical bright white. Which can be a bit tough on the eyes at night. You can have it theme the launcher's settings (not the device settings since that isn't part of the launcher), the app drawer, the search bar and folders. Nova also allows you to decide when the launcher goes into night mode. You can have it set to sync with the sunrise and sunset, or enter your own times. Or if you really don't like the white color, keep it in night mode all of the time.
Other items in the changelog include some bug fixes and some improvements to the search bar configuration. So it's not that big of an update to Nova Launcher, but those that love night mode, are going to absolutely love this update to Nova Launcher. You can install Nova Launcher from the Play Store, as mentioned there is a paid version known as Nova Launcher Prime which costs $5 and unlocks a bunch of other features.