Android Messages has been updated to version 3.5, bringing dark mode and a new user interface that fits with Google's new Material Theme Engine conventions. The new UI is pure white and uses the Google Sans font, but can be switched over to dark mode with an entry under the menu button in the top right. Unfortunately, on top of the new features comes the removal of one that many users valued; the app's widget. You'll no longer be able to peruse messages on your home screen, it would seem, and those who liked the ability to assign each contact a personalized color scheme will be dismayed to find that the minimalist UI has done away with those colors, and thus that capability.
Under the hood, everything is mostly the same. The app's core function, RCS messaging, is still intact in the same form it's always been. You can send and receive all kinds of content right within the app, without having to open anything separately. The new update may not have hit you just yet, since it's currently making a slow rollout through the Play Store. If you can't wait to get your hands on the newest version of Google's flagship messaging app, you can always dig up an APK file on the internet and sideload it, but keep in mind that doing so could turn sour in a number of ways.
Android Messages is now finally jumping on board with Material Theme Engine, often referred to as Material Design 2.0. This seems to be the first fully Material Theme Engine compliant RCS messaging app out there, though it's entirely possible that an app created from the ground up for compliance is out there somewhere. Many of Google's other apps have already made the jump, such as the main Google app. Android Messages, meanwhile, picked up smart reply, GIF sharing and a web client in its most recent update before this one.