You can change the way many things look.
1. Click on "Design".
2. Click on one of the Cover choices (Minimal, Side-by-side, or Full) to change how the top image and the title appear.
3. By clicking on "Navigation", you add the main headings to a strip below the top banner as navigation links.
4. You can change the way that backgrounds and maps appear by clicking on one of the Theme choices. For example, Mesa gives your StoryMap an old-time look with sepia coloration while Ridgeline (shown below) gives your StoryMap a greenish background with parklands showing up green on your Express maps.