When developing a custom theme from scratch, is there anything I need to do to ensure Facebook Pixel / Conversions API partner integration works correctly?
This is PageFly - Free Landing Page Builder. I would love to provide my recommendations for your store.
Regarding your concern to make sure that Facebook Pixel / Conversions API partner integration works correctly?
You can install the extension for your browser name: Facebook Pixel Helper on browser app store
After that you should follow all the steps from set up and add the ID to your store preferences.
Then you can check it on your live page, if your pixel is running properly, you can see this event fired and tracked from the tool like this
Hope this helps.
Best regards,
PageFly