Just follow the steps for Debut theme for Add to cart button on collection pages.
- From Shopify Admin select Online Store and click on Themes
- Choose your main theme
- Click the . . . on the top left-right (see below) and hit Edit code
- Open Snippets -> product-card-grid.liquid File and paste below code at the end of the page
<form method="post" action="/cart/add">
<input type="hidden" name="id" value="{{ product.variants.first.id }}" />
<input min="1" type="number" id="quantity" name="quantity" value="1"/>
<input type="submit" value="Add to cart" class="btn" />
</form>
