How to create products with variants that have separate stock levels

Hi,

We run a clothing/merch store for teams/brands and are looking to introduce stock levels as currently we print on demand. The product specifically we are trying to customise is this one here https://dwpromo.co.uk/collections/lions-player-teamwear/products/offer-kids-edge-pro-training-tee-embroidered

As you can see we have the option for the customer to select size, this is ran from an app called ‘variant option product options’ - we are wanting to show independent stock levels for each size, i tried following this tutorial https://parasoul.ltd.uk/shopify/dawn-theme-display-inventory-quantity but hit a roadblock on the last step (4. Open the global.js file and search for the toggleAddButton function and replace it with this code), i am unable to do this as i cannot locate the Global.js, the closest i have found is ‘theme.js’ which does not have the toggleAddButton function inside? Is anyone able to help?