Is there a way to set the default font?

Just look at the Settings–List File. You will see a New Text page where you can set up the font to use for any new topics that you create. But, the default font for the existing topics can’t be changed as they already contain the rich text encoding. For this, you need to select those topics and do the right-click action Change–Change font.

You can also change the font for List of topics on Settings–List–List Settings page.