Themes, checkout submodules #2

A number of themes suggest we add a submodule within the "themes" folder.
But then the github action described here does not pass because the theme is not loaded.

The fix is simple once you know it:

      - name: checkout
        uses: actions/checkout@v6.0.1
        with:
          submodules: recursive

Then it works just fine (thank you very much by the way).
Now if you don't want to add that in your README, it's okay, feel free to close this.