There's no way to set the font in design mode, but we do have an advanced undocumented command called
setStyle that allows you to assign a CSS style to your design mode elements (great callback to unit 2!).
setStyle takes two parameters, the first is an ID of an element, and the second a string of CSS to apply to that element, for example:
setStyle("myButton", "font-family: cursive; font-weight: bold;");
Here's a quick example app showing how
setStyle works https://studio.code.org/projects/applab/YzXR4wJXjGfrUSuFp9qzYA/edit