How do I change the default font for the presentation that I am preparing?

I have found the template I want to use, but I want to use a different font. Every new slide still has the old font. How can I set a different global default font for my presentation.

I have tried the inspector among other things, but can't find anything. Setting the font every time I want to type is getting a bit tedious.

If you know the solution it would be really appreciated. (I am new to Keynote.)