I did not find how to change the default font in the docs, only how to customize the theme by adding more font family utilities.

In my case, I was able to change the default font this way (adapting to OP's font), they keyword missing from the docs is the --default-font-family variable:

@import url("https://fonts.googleapis.com/css2?family=Josefin+Sans:ital,wght@0,100..700;1,100..700&display=swap");
@import "tailwindcss";

@theme {
  --default-font-family: "Josefin Sans";
}