Is there a way to apply a single font to all elements in a keynote presentation?
Changing the font used in a theme maybe?
If you open up the master slide for your presentation (view->show master slides):
It should display a series of master slides in the sidebar:
For each master slide your presentation uses, you will need to do the following steps:
Unfortunately there doesn't seem to be a more efficient way of changing every font, as there is no way of selecting content across multiple slides.
If you have created text boxes outside of the slide layout, you will also need to manually change these aftwards.