Board Thread:CSS Development/@comment-25744472-20191230072513/@comment-24692230-20191230073757

You you're using the old version, not the modern version, you need to set  in your wiki's JS for it to show the modern button, which then your CSS will affect, also, unless you import MediaWiki:BackToTopButton.css it won't affect anything, so you can delete it and just keep the CSS in your Wikia.css.