Re: Why has my settings screen background turned white. I want the black background back
Welcome to Android Central! Which phone, and which Android version? It's probably a change related to Lollipop, which introduces a lot of light colored backgrounds. You can't change it unless you want to try various alternative launchers like Nova Launcher.
There have been rumors that Google will introduce a dark menu theme in Android 5.1, but nothing substantiated yet.
Re: Why has my settings screen background turned white. I want the black background back
A quick comment. With Nova and such you can change the drawer background. But I haven't found a way to change the settings background. I think you have to root and flash a custom rom or a theme for your phone (if available already, of course).