A week back, Google had updated its Messages app to add the Material UI, new GIF Search and a Dark Mode, but took it down in a server side update.
But it’s now back in the latest version — and you can download it now.
After downloading the APK from the link below, you just need to Force Stop the App to get the design live in the app. And to enable the dark mode, just tap the three dots at the top right of the app and select “Enable dark mode.” To disable it, tap those three dots again but choose “Disable dark mode.”
Download : Android Messages
or you can update it from the Google Play Store.