Hey there @Wesley - Solid Squares!
Welcome to the Community!
At this time, we are only able to help implement back-in-stock for standard free Shopify themes. You can see the basic free themes that we support listed here: Back in Stock Styling Code Snippets for Free Shopify Themes
The reason why we do not support all custom themes is because theme developers are not required to follow some basic guidelines that allow our back-in-stock feature to function. Sometimes the theme developer will be able to help with this implementation or you can reach out to one of our approved partners here.
If you end up working with a developer, you also have the option to build a fully customized solution. Our Back In Stock feature works by sending an event metric into Klaviyo which triggers a flow. You just need to send the Variant ID, Product ID, and email address. All of this can be custom-built using our open API to work with your website. Here is the necessary documentation for that: Shopify Back in Stock for Developers
Otherwise, there are other back-in-stock solutions supported through Shopify that you can find through their apps section. Here is a link to their site.
Please do let me know if you have any further questions or check out our handy Help Center.
Hope you have a great day!
Alex