Google has updated Play Movies & TV with a dark UI theme as part of their most recent app update, although this doesn't really seem to be a theme, rather the background color of the user interface is just switching from white to black. This might seem a bit strange as many of Google's other apps have a unified white background color to keep the designs between them as streamlined as possible, while still holding onto their own unique design elements. That being said, depending on how you feel about the color scheme, Google's new use of black for the background color might suit your tastes.
If you think about it, the black color for the background kind of makes sense. The app is about movies and TV, and when viewing a movie or watching TV you probably want it to be somewhat dark around the TV screen so you can watch things in a cinema-like fashion. Perhaps that's what this design change is attempting to mirror, and perhaps not. In either case, one thing is for certain and it's that Google Play Movies and TV looks a whole lot more like Netflix now, as they changed their app to a black colored background and UI theme some time ago, and of course their themed color has always been red.
The app update seems to be rolling out to users as of right now though there is no telling how long it might take to reach all users, so you'll just have to check and see if the update is available and download it if it is, or simply wait patiently for the update to hit your device if it's not. The new black color can be seen throughout the app on the My Library pages and on the Watch Now page, and Google has completely gotten rid of the red action bar up at the top of the app. Whatever Google's reasoning for removing most of the red accent colors in the app, it does seem to put more focus on the content itself, as without the red there is less stuff to draw your attention away from what should be front and center, and that's the movies and TV content that the app contains. The app that has these changes should be version 3.22.