1. In a chat window, open the ⋯ menu and choose Chat background.
2. Pick a background: Wiply, Sunset, Lavender, Night, Mint, Sky or Paper, or None for a plain one. The preview at the top shows how your chats will look.
3. Under Use it for, choose This chat or All chats.
4. Click Apply.

Each built-in background has its own version for Wiply's night mode.