Hi CartoFred,
You can swap in different fonts if you download the code for the Story Map Cascade app from GitHub and host the app yourself. Here's an article about how to host a story map app on your own server.
Cascade currently uses Noto Serif for the narrative text and block quotes and Open Sans for the cover, title sections, immersive section titles, media captions, and H1/H2 headings. Open Sans is also used throughout the builder. If you change these we recommend swapping Noto Serif for another serif font and Open Sans for another sans-serif font.
You'll need to update the font references in the app code and, if you aren't using system fonts, add your font files to src/app/storymaps/common/_resources/fonts/.
Owen
Owen Evans
Lead Product Engineer | StoryMaps