It's easy peasy lemon squeezy to set the default font for text boxes in StoryBook Creator 4.0 (US|CAN). Go to Help (it's that little question mark icon in the top right corner of the screen) and select Options. Click Text style. Choose the font, font size, justification, color, etc., that you'd like to use as the default, and click OK.
Ta da! All text boxes added to projects moving forward will use those text attributes by default.