Google Messages introduces chat themes replacing old 'Change colors' feature

Google Messages beta themes apply locally

You can upload your own wallpaper or pick from Google's collections: think Animals, Space, Sunsets, and more.
Right now, it's only available to some beta users as part of a phased rollout (look for a red dot and "Chat theme" in the menu if you have it).
These changes are just for you: your friends won't see them on their end.