WhatsApp has finally been updated to the iOS7 style. Unfortunately the backgrounds in the app don’t suit that style at all. Here is a much better background for your WhatsApp chats.
In my opinion a light gray works best.
Just tap and hold and choose the option to save the image to your camera roll.
In Whatsapp go to Settings > Chat settings > Background, and select the background from the camera roll.
Here’s a white one (the border will not show up in the image, it’s just to indicate where you can tap and hold).