If you're not using a chat mod like Prat or Chatter then open up your nUI localization file in the AddOns/nUI folder and locate the one for your locality. There will be 2 sets of nUI["Font1"] nUI["Font2"] .. names from memory so forgive me if they are slightly wrong
.. One will affect chatter and the other for pretty much anything else.
You will see one line will not have -- in front of it. This is the one in current use. Simply put -- in front of it and take out -- in front of one of the others in that block and do a /nui rl and see the result.