To create a new design, please follow these steps:
Click on the "Design" button on the left side of the page:
In the setting, you will have few tabs: Themes, Product design, Custom CSS.
In the themes section, you can choose the theme for your offer.
In Product design section you can edit and customize: Product background, Product border, Product title, Product price, Product button background.
In the Custom CSS, you are able to write your custom code for your offer.
Don't forget to click 'Update' button