1 Make sure you are on Customize tab

Most of sections have background blocks, click + under any section and check if there is block starting with "Background ..."

Choose any background block, for example "Background Gradient" and fill it's options on the right side.

You can also add more background layers, as example we added another "Background Gradient" in Joosie theme and made one color transparent and then changed angle.

Backgrounds in Shopsta are incredible and it's all about your imagination so get creative