I think this option is available in this theme by default.
Sorry my mistake.
To make add to cart sticky you need to add JS with logic on scroll to specific position.
You can check example here
And you can add this code in existing JS file in theme.liquid before body tag (or you can create you custom file and include it in theme.liquid)
Also to make it full width in footer, you have also to add some css a/to your need.