Over the past few weeks, Google has been changing the interface of a few applications, making them brighter with more rounded edges. The mobile search site recently switched to this layout, and the in-progress Chrome redesign has it too. Assistant has now fits in with this design language, as a visual change has started to roll out to users.
Left: Old design; Center, Right: New design
As you can see in the above screenshots, the background is white instead of a light gray, and the buttons now have a slight shadow.
Google updates Assistant interface with white background and rounded cards was written by the awesome team at Android Police.