To change WhatsApp chat themes, go to Settings > Chats > Default chat theme and pick from preset themes or upload a custom ...
WhatsApp is adding new choices for customizing your chats, with both preset themes and mix-and-match colors for backgrounds ...
WhatsApp Is Rolling Out Chat Themes, Letting You Customize Your Chats With Colorful Bubbles And New Wallpapers! (Personalize Your Chats) ...