Board Thread:Support Requests - Designing Your Wiki/@comment-26868869-20170621033253/@comment-957747-20170622161109

Looks like the staff that replied misunderstood your question. The question was asking if custom fonts were allowed. It was not specific to the new header.

Yes, a wiki can use custom fonts. No, wikis cannot adjust the font-size, font-family or otherwise within the new header. If it's not allowed within the ThemeDesigner, it's not allowed.