hey, @andersmellerup
To do that add below css into Appearance → Customize → Additional CSS:
#footer {
position: fixed;
left: 0px;
bottom: 0px;
right: 0;
z-index: 9999;
margin: 0 auto;
width: 1040px;
margin-left: 440px
}
But I suggest you to use Sticky Side Buttons plugin instead of sticks the full footer section. It looks like screenshot and you can also change the position of this sticky social icons and this plugin also provided some functionality for designing and all.
Hope the above will help you.
Thank you.