Open the Messages app
Open the conversation you want to change
Tap the chat name or menu at the top
Look for Wallpaper, Background, or Chat theme
Select a background or theme
Choose a color, image, or preset
Tap Apply or Set
Save the changes
If you do not see this option, your messaging app may not support custom backgrounds
On iPhone, use iMessage effects or wallpaper settings in the app if available
On Android, check your Messages app settings or chat customization options