Customizing Add to Cart Option on Product Grid Page

Hi

We are building our erpnext front-end by customizing the exisiting erpnext theme.
Need help with customizing the "ADD TO CART BUTTON’ on Product Grid page.

Currently the Add to Cart Button doesn’t have the option to choose Quantity. That is via clicking + or -

for reference pl refer to the attached image.
add-to-cart

Can someone help on this?