Go to the messaging thread of the person who you want to change the bubble colours of, press the 3 dots on the right side of the screen and in the drop down menu select change colours and you can change the bubble colours.
As for the wallpaper I don't believe it can be changed, hope this helps.
Well Google messages has the option to use RCS which is like the Android version of imessenger. That's why you don't have those options unfortunately. Maybe try using a different messaging app like Samsungs etc if you don't want RCS.
"1. Open Google Messages. 2. Select a chat you want to change the theme. 3. Tap More: Change colors. Change colors. For chats with an individual contact: Tap Contact name > For group chat: Tap Group name > Change colors. 4. To preview a theme, tap the color from the selection. 5. To select a theme, tap Confirm"