You can enter variation theme overrides at channel level by entering custom fields. This way, all products’ variation themes will be overridden by the channel level variation theme override. This is useful, if you want to send a prefix variation theme for all products rather than using already existing variation themes for Shopify.
The setting is located at “Channel>Channel Settings>Shopify Settings>Product Data Settings”.
You can select different custom fields from the dropdown.
IMPORTANT NOTE: This settings will be applied for all products in Shopify channel.