This new update in design only adds one new feature: a dark theme option in the settings. At the time of writing this, there is no way to enable that feature. For some that might be a great feature because of all the white. The design updates are, in my opinion, the best design that is currently used in any app. Below are some side-by-side comparisons between the redesign and the older version.
In my opinion, this update is probably the biggest and best update Android Messages has gotten. I always switch between Samsung Messenger and Android Messages, but once this update goes live I will instantly switch to it. This new design is a great upgrade over what is currently used in Android Messages. I am going to really enjoy seeing other apps and devs update their apps to use this same Material Design 2 design.
How to enable Material Design 2 on Android Messages
Install MiXplorer from the XDA Labs. This will be required for modifying the shared_prefs settings stored in the data partition.
Open MiXplorer and go to the root section. Once in root go to /data/data/com.google.android.apps.messaging/shared_prefs/. In this folder open PhenotypePrefs.xml. When the popup opens to select editor, select code editor.
In the top right corner, tap the three dots and select find, search for bugle_phenotype_enable_m2. It will find this boolean.
The value should be set to false at the end. Set this to true.
Go back to the find option and find the option bugle_phenotype_enable_phenotype_override. You are also going to want to set this true.
Once you do this, force close the Messages app.
Once you re-open the app you will see the Material Design 2 redesign. The only problem with doing this is that this redesign will only last once per force closure of the app. If you close the app out of Recents by swiping it away, it will be reset to the original design.