Sticky Footer

Has anyone been able to get a sticky footer working?

Obviously, there are lots of solutions for sticky footers available online, but most presume that you are starting with fresh code. It’s not clear what classes the css changes would be assigned to within the theme.

I have tried flexboxes and a number of other things. The closest I got was with the code at https://mystrd.at/modern-clean-css-sticky-footer/

This definitely stuck the footer to the bottom of the viewport, but did not extend the content box down to meet it. I am attaching an image of the current footer (without any css modification).

Anyone got a good idea?