1. Launch the Site Builder tool from your cPanel.
2. Click on the Settings icon at the top menu and choose Background.
3. Choose what page you want to apply the background image/color to.
4. Click on the 'Pencil' icon under Image to upload a background image or click on the 'Pencil' icon under Color to change the background color.
You can also set the position of the background image or set it to repeat horizontally or vertically. You may add a secondary background by clicking on 'Add other Background over this?' link.
5. Click on 'Apply' button.