If you want to customize the look of your app, you can do so from the Settings menu. 
Simply go to Settings – Background and either:

tap on “Select colour” to choose a solid background

tap on “Select a picture” to use one from your phone Gallery (you might be asked for permissions if you haven’t given one up to this point). Please note that the app automatically rotates pictures in portrait mode, so make sure the photo you use is already in portrait!

tap on “Download old backgrounds” if you are nostalgic for the one you had in a previous version of the app. Which is totally understandable, we had some very suitable ones 😉