Change the Instagram DMs Theme
The themes are available on version 165.0 so make sure your app is updated to this version. If it isn't, you'd see an update on the App Store. Once the app is updated, open the Instagram app and tap the Messages button on the top right. Read: Here's How You Can Change Instagram App Icon
Open a chat and tap the details button on the top right. On the details page, you'd find the Theme option, tap to reveal the available themes.
The themes change the background as well as the chat bubble color whereas items listed under Colours and Gradients only change the chat bubbles. Select the theme of your choice and voila, you've successfully changed the theme of your Instagram DMs.
The good thing about the themes is that you can have separate themes for every contact and when you change a theme, it also gets changed for the other person in that chat.
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Updated: November 28, 2021