FWIW I also worked this out ages ago and then forgot how to do it
I reworked it out just before going on holiday and so am again a bit hazy but I think I managed to change the Viewport Title fonts like this.
Open from your Annotations folder within your Imperial or Metric Library the family called View Title.rfa
(there are other related families with other options but I think this is the default one).
Once that family is open, highlight the label you want to change font/style etc and click the Properties button, then Edit/New and Duplicate and create a new named Label style with your choice of Font, which can be chosen within the Properties dialogue.
When the family looks like you want it Save As... out and/or Load it into your appropriate project, if that is Open.
Now in a drawing sheet highlight a viewport, click Properties and Edit/New and Duplicate, create a new named Type, and within the Properties dialogue click on the Value called 'View Title' related to the Graphics Parameter called 'Title' (it's top of the list) and you should see your newly created family on the drop down list.
Select it and that should now make the View Title take on the desired font.
There may be a tidier way to do it but that seems to work for me.