This is a premium feature available with Astra Pro Addon plugin. To use these Pro features, you need to have the Astra theme along with the Astra Pro Addon installed on your website.
Please refer the following document, to begin with, Site Layout.
Technically, Padded is just the Full-Width Site Layout, with extra options for the padding across the website edges.
The padding remains consistent even when user scrolls and thus, it results in a neat frame kind of design for the website, seen on many creative websites.
You can find the settings in Appearance > Customize > Global > Container > Site Layout
Note: Padded layouts work best with Full Width / Stretched.
You can manage the padding independently for different devices as we’ve provided that flexibility.
Here is one of the Astra Demo Websites where we have used the padded layout option:
https://websitedemos.net/plant-shop-03/
Other Notes:
If you need to set a background image for the entire site please check this document: Global Colors – Astra Theme.