You can set up a chat room’s wallpaper screen.
After entering into a chat room you want to change the background, press More options (on the upper right side) - Settings (on the lower right side) - Wallpaper Settings.
When changing your background, if an artist has changed background, you might not see the artist's set background.
(You can only set to the same background as the artist's by setting it to “Change to bubble background.”)