Board Thread:Support Requests - Getting Technical/@comment-26447726-20190130015003/@comment-34804319-20190130133804

You are using  on your common.css page, which will change the font on the entire page to your selected design. As per the customisation policy...

"Most notably the global navigation bar (top) and the footer at the bottom of every page. This is the basic framework of the site, containing useful links for all types of visitors, and must retain a consistent appearance across the network. You also may not change the color or formatting of the FANDOM logo."

... you may not change anything regarding the styling of the top navigation bar.