Is there a way to have a drop down list for choosing amounts to add to cart?

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My theme is Debut. I tried using "show quantity selector" when editing the theme, but that is worse. It just turns red if you try to add more products than I have in stock. My stock varies widely and customers often buy multiples. I need them to have a way to see clearly how many they can buy.

Is there is an app for this or a code that is easy to use. I want my product pages to have a drop down menu where customer can choose how many to add to the cart. I am using Debut theme.