Board Thread:Support Requests - Designing Your Wiki/@comment-38741379-20200409022717/@comment-44807868-20200409041822

Actually, that wiki is using UCP so CSS can't be used there yet.

As for the issue, I tested it out on UCP, too, and customizing border width settings appears to be not working quite right... A bug, I assume. For now, you can try removing the border using source editor instead: