A couple of days ago, Google has started rolling out Android Messages v3.5 with Dark Mode and new design, amongst other improvements, but it seems like the company quickly backtracked, and removed said dark mode from the app, along with the design. This was spotted by a Reddit user, who complained that Android Messages reverted back to the old design quickly after the update hit with new design and dark mode. And no, a new update did not land in the meantime, Google simply rolled back to the old design via a server-side change.
Android Messages v3.5 originally brought the company's new Google Sans font to the table, along with a number of other visual changes, such as removing the blue bar that was included at the top. Android Messages v3.5 basically had a lot of white color going on, or dark gray / black in dark mode, until Google reverted back to the old design. This design was rolled out and quite probably will be again in order to fit well with Android 9 Pie, as the company decided to redesign Android Oreo, and release a new version of Material Design, basically. Reactions to the new design are conflicted, some users love it, others think that it does not have anything to do with Material Design, and that Google is going the wrong way. Whichever the case may be, this is the design that we'll all be seeing in Google's apps in the near future.
It is still unknown why Google reverted to the old design so soon after updating Android Messages, but it's possible that the company stumbled upon some bugs or something of the sort, and that it wants to iron those out before making this new design and features available to everyone. In any case, you can expect to see that design rolling out to Android Messages once again in the near future.