How to set the default font in a beamer sty-file?

Question

I'm currently creating a beamer sty-file and want the style to change the default font. I tried the following without success:

1. \setmainfont has no effect.
2. For \setbeamerfont{<type>}{family=XXX} works for types listed e.g. here What are all the possible first arguments to `\setbeamerfont`?. But which type to use for the default font? According to the beamer user guide normal text has no effect yet.

So, how to set the default font in a beamer sty-file? If this matters: I'm using Luatex.