Post purchase offers can not be designed as the page is rendered on Shopify's end. The offer page will automatically inherit your checkout page design (fonts and button colors)
For all other funnel types:
1. Open the "Design" tab to edit everything design related int he app.
2. Choose a pre-made template to start with by clicking on one of the quick themes. It would be easier to change when you get a close match to your expected result.
3. For further custimization - scroll down the page and use the left side settings cards.
* If you want to use your store color palette - click the color picker and choose colors below "store colors" section.
4. Click "Save".
The app isn't working as expected? > let us know.