First spotted by a Reddit user, the change has been confirmed by many whose app reverted to the default, white background and locked them out of the setting to switch to Dark Mode. We can also confirm that the same happened to us on multiple devices, upon checking after seeing the post.
We have no information currently on why this is happening, as Google did not immediately respond to our request for comment. Dark Mode on Android Messages can still be enabled on rooted devices, but Google has obviously pulled the theme for a reason. The reversal may be temporary, however, and made with the intention to solve bugs. We wouldn't be surprised if Google flips the switch and enables Dark Mode again in the coming weeks.