Board Thread:Support Requests - Designing Your Wiki/@comment-1807002-20191118023840/@comment-168424-20191119190334

You should experiment in Special:MyPage/wikia.css and not the wiki-wide CSS page. The wiki-wide CSS page should only be used for tested and known working CSS.

The link to your CSS page has no usage of last-child in it that I can see. You also deleted the sample page.