Is the Retina theme currently live on your online store under Online Store > Themes > Current theme, or are you working on it in the background under Online Store > Themes > Theme library? I've noticed that newsletter signups through the theme editor or preview of the store might not save from a theme that isn't live. Alternatively, they may just take some time to show up in the admin.
If Retina is the live theme on your store, any newsletter signups—from either of the theme's newsletter signups (separate section or footer signup) or the online store's checkout—should appear under the Customers section of your admin under the Email subscribers tab. You can also click on a customer's name to view their profile, and whether they've subscribed to emails will be listed there too.
Let me know when your last signup attempt was and what you're seeing in the admin now. I'm happy to take a closer look into this with you if the newsletter signups still aren't functioning as expected.
Sophia | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog