Product cards can be styled by going to Customize > Product Catalog. Here you will find the Style tab, click on it and then find Product card section.
Once you click the Product card, you will find the following options:
You can choose one of the three available card styles or use the radius controls to round your cards and product images. Color options are also available for the card background and product title and you can also customize the border size and color. Please note that the Card Style option is available only in Sydney Pro.