There are two different design options for the style of thumbnails - small and large.

Small Thumbnails: 

Big Thumbnails:

To change the settings, you have to go to the Manager and follow the steps:

1. Click on Content

2. Choose the Publication, which you want to change on the right-hand side

3. Than a pop-up shows up where you click on Edit 

4. A new window shows up. Now you scroll down to the point you see the TOC-style settings. Here you can choose between small and large thumbnails.

5. Additionally, you can also hide the page numbers

6. Finally, you have to rebuild the app to make the changes visible.