Random div appearing

I’m trying to create a custom theme based on a blank theme (https://www.siftware.co.uk/services/wordpress/blank-wordpress-theme/)
however, after activating a few plugins and modifying the css, a random box has appeared and is pushing my sidebar down.

I’ve been using developer tools in Chromium to try spot where it is coming from, but its showing up in every element…

Any help to get rid of it would be appreciated.

URL: https://blackinternational.hasahappy.me.uk/