In the OPAC preferences chapter, under OpacAdditionalStylesheet (https://koha-community.org/manual/latest/en/html/opacpreferences.html#opacadditionalstylesheet) "enter the full URL pointing to it’s location" it's -> its Please also review this whole sentence. I think there are missing words and puctuation. "If you would rather, you can upload your CSS to another server and enter the full URL pointing to it’s location remember to begin the URL with http://"