You can choose to use a color or an image for website background as the following guide.
1. Website Background Color
In Step 2, click "Website Settings" -> Color Picker
Then select the color tone for your website background, and press "OK".
Then preview on the right hand
2. Website Background Image
In Step 2, click "Website Settings" -> “Advanced”
1) Click “Choose from Image Library”, and select an image from your computer.
2) Click on the uploaded image.
3) Set how your background image repeat.
Then preview on the right hand.
Everything you did in Step 2 , please don't forget to save by "Update Template" button at the top of step2.