User blog comment:Rupert Giles/Global Navigation January Update/@comment-11842624-20150218135711/@comment-1251315-20150218140500

No, the navbar is Wikia's space so naturally any changes there MUST be in personal JS/CSS. Site-wide changes via common.js/css etc will just be removed by a staff member, but you are free to basically do anything you want to the header in your own global.css file. :)

EDIT: I could be wrong though, so definitely wait for a staff member to let you know if it's customisable.