Hey there,
You’re using the original X footer and not the footer builder. Anyway, you can achieve that by setting your footer bar’s position to absolute, add an ID and target it to have a bottom setting of 0.
Here is a sample of how it would look like.

The custom CSS used for that is this:
.x-colophon.bottom {
background-color: transparent !important;
position: absolute !important;
bottom: 0 !important;
}
Please note though that I am not encouraging the use of custom code unless you’re knowledgeable with it. Also, we do not support issues arising from the use of it. I gave that for demonstration only of what you can achieve with controls in Pro.
Thanks.