WhatsApp has released an update featuring chat themes, allowing users to customize their chat conversations with colors to add to their background and chat bubbles, along with 30 options for their wallpaper to further personalize their conversations.
You can choose to make your own combo of colors and themes, or you can choose one already made from preset themes. The new choices added provide freedom to customize separate conversations with different settings, and these can only be seen by the person who set the theme.

This is accessible by going to Settings, then Chat, and then choosing Default to set the theme across all chats for a unified look. To do it separately, just go to the chat name on top of your screen to apply the theme to that individual chat. The update will be rolled out to all users of WhatsApp in the coming weeks for everyone to access the new feature.