Why do i have a red banner/ toolbar suddenly?

[AC Question]

Completely unprompted, my android now has turned the black bar at the top and bottom of the screen to red instead (the bars with the time at the top and home button at the bottom.) It's a Google phone and I installed the latest update a few days ago. Tried restarting, it's still there. Why did this happen? And more importantly, how do I change it back? Thank you!