The CAS background is so very boring in The Sims 4. I used to use a plain colour background so that cropping around the sims was simpler, but I have since discovered Ellcrze and their amazing CAS rooms. Be sure to only add ONE of these background files to your mods folder, but you can switch it out as often as you need for amazing CAS screenshots.
I recommend using a blob remover mod to hide the shadow created by your sim.
All the sample images below are (C)Ellcrze and are not a representation of all the backgrounds offered, these are just some of my favourites.
All of Ellcrze’s backgrounds can be found for free on their Patreon. To my knowledge, at this time, Ellcrze has stepped away from creating to focus on her new role as a mother ♥