User blog comment:DaNASCAT/Technical Update: June 12, 2017/@comment-25529993-20170616055517/@comment-20644-20170618231140

The only exception for customization are icons like in BlondeLegendaire's screenshot. We've allowed those because they are a fairly typical part of wikis dating back many many years to Wikipedia. The only alternative to the header would be to place them into the content area, which just doesn't look very good.

If your JS for that was denied, use Special:Contact to ask the Community Technical team why it was denied.