For a simple HTML footer, you can do that with our theme system. On your site, visit /admin/customize/themes. Select the active theme, and click Edit CSS/HTML. Within the theme editor, switch to the “Footer” tab and add the HTML you’d like. Click save, reload your site, and you should see the footer. Note that the footer will only appear when scrolled all the way down, so you may wish to check that it worked on a page without infinite scroll like /categories.