Board Thread:Support Requests - Designing Your Wiki/@comment-38546904-20191019141103/@comment-9605025-20191019221808

They just use CSS for that. The exact CSS they use can be found on MediaWiki:Common.css. However, I have provided a modified version below. The modified version should not impact Tabber tabs and also avoids the bad practice of useing "!important"; which they do quite a bit.