The upsell window has some standard styling and pulls some styling from your theme's defaults. You can override this by following these steps:
- From "Shopify Admin", select Apps.
- Select Product Upsell.
- Select Upsell Settings.
- Scroll down to the "Custom Design" section.
- Enter the CSS needed to override the existing styles.
Note: To find the styles you can right-click on any element and select Inspect or Inspect Element. If you're not sure how to use the browser tools visit: Chrome Developer Tools.
- Select Save.