Shopify Sold-Out Label Update

My store runs on Shopify and I’d like a small but important tweak to the product page: whenever inventory reaches zero, the page should automatically show a clear “Sold out” notice. I want this shown as a simple text label—no greying-out or hiding the item—styled with a black underline so it catches the eye while keeping with the rest of my theme.

Please add the necessary Liquid, CSS, or theme-editor changes so the label appears the moment stock hits 0 and disappears again as soon as items are restocked. Everything else on the page should stay exactly as it is; I just need the sold-out status handled reliably and cleanly.

Hand-over can be a quick theme file update or a documented snippet I can drop into future themes. Let me know if you need collaborator access to the store so we can keep this simple.