Someone's background:
1. Open the chat window with friends, click the three points in the upper right corner to enter the chat setting interface (as shown below):
2. Find "Set the current chat background" in it (as shown below):
3. Click the following three ways to change the chat background (as shown in the figure below):
Background of all friends:
1. Click the "Settings" function in the "Me" interface (as shown in the following figure):
2. Click the "General" function (as shown in the figure below):
3. Click the "Chat Background" function (as shown in the figure below):
4. Just choose one of three ways (as shown in the figure below):