My back camera of iPhone 5s isn't working. It is showing a dark background.

Hello,

You see a closed lens or black screen

If you see a closed lens or black screen when you open the Camera app, try these steps:

1. Make sure that there’s nothing blocking the camera lens. If you’re using a case, try removing it.
2. Force the app to close, then open the Camera app again.
3. Restart your device, then open the Camera app again.
4. If your device has a front and rear camera, try both cameras by tapping . If you see the closed lens or black screen on only one camera, take your device to an Apple Retail Store or Authorized Service Provider for more help.

From:

Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support

iPhone 5s back camera isn't working.How to get back that?