You can add a background to your website globally in Jupiter X.
To add a background image/color to the whole website:
1. From the WordPress left dashboard, go to Appearance > Customize.
2. From the Customizer left dashboard, go to Site Settings menu.
3. In the Styles tab, click on Customize for Main.
4. Now you can set a color or upload an image for background.
5. Click on x, close the Site Settings window and then Publish the settings.