Open the Messages app
Open the conversation you want to customize
Tap the chat settings or menu icon if available
Select Background, Theme, or Wallpaper
Choose a preset background or image
Apply or save the changes
If your phone does not support message backgrounds, use a messaging app that does
On WhatsApp, open the chat, tap the menu, then choose Wallpaper
On Telegram, open the chat, tap the menu, then choose Change Background
On Samsung Messages, open the conversation, tap the menu, then choose Wallpaper or Chat background
On iPhone, use iMessage effects or change the device wallpaper, since custom message backgrounds are not supported in the Messages app
Update the app if the background option is missing
Check the app’s theme or appearance settings
Restart the app after changing the background