There are some options depending on what you want to change. For something like the title or subtitle you can use html/css in the text box to change the font. for example you could do something like this for your title:
<span style="font-family: fantasy">Fantasy Title</span>
if you need more control, a few of the templates (including Basic Viewer and the new version of Elevation Profile) have a custom css parameter which allows you to override the style of different elements in the app.
Did you have any specific thing that you wanted to do?