How to change the background color of your sign-up page

Open the app (Shopify admin → Apps → ConvertOut), go to the Settings tab, and scroll to Portal options.

Enter a hex value in Background color — for example #F7FAFC — and click Save.

Portal options settings showing the Primary color and Background color fields

Primary color sits right above it and controls buttons and accents, so it's worth setting both to match your brand at the same time.