How does oppo r7 set SMS and contact background?

There is no such setting in the mobile phone system settings, so you can't set it yourself.

This can only be changed by using the global theme. Go to the software store to find it. There are few global themes, and even fewer can be completely globalized. Just recommend a global theme, impression space, screen lock wallpaper or desktop wallpaper that you don't like to change. I also use r7, which is the theme I use now.