To add CSS to the Product Upsell window to make it responsive on mobile devices, follow these steps:
- From "Shopify Admin", select Apps.
- Select Product Upsell.
- Select Settings.
- Select Upsell Settings.
- Scroll down to the "Custom Design" section, and copy and paste this code into the Custom HTML/CSS text area: Product Upsell Window Responsive Code.
- Select Save.