Board Thread:Support Requests - Designing Your Wiki/@comment-44997843-20200614165740/@comment-9605025-20200614175307

In general, other than the article content and right rail, you are not supposed to change anything on the page using CSS. This includes the global navigation bar, the local navigation header (the part you are asking about), and the page header (the part with the page title and edit button). - Edit:

The exceptions are whatever the provided tools aside from CSS/JS allow you to do and what is explicitly listed on Help:Customization_policy.