If you trust WhatsApp as your message, you're probably too familiar with the default background in chats. While its distinct appearance definitely sets it apart from other messaging apps, you're free to swap it for something more appealing.
To do so, open WhatsApp and go to the app's settings. If you are on an Android, tap the three-point menu icon and select "Settings". If you are using an iPhone, just click the "Settings" icon on the main screen. Either way, select "Chats" and then press "Wallpaper" (Android) or "Chat Wallpaper (iPhone).
From there you can choose between different images and solid background colors. You can even use a photo from the phone gallery. If you are using Android, you can also install the WhatsApp Wallpaper app for even more choices, no matter what image you choose, you will finally get rid of the old default wallpaper!