maybe you should modify the code and insert the - button before quantity_wanted and +button after quantity_wanted input
in product.js ? or other js files. i don't view the code in fact
it seems this code in product.js default theme
function createProductSpin() {
const $quantityInput = $(prestashop.selectors.quantityWanted);
$quantityInput.TouchSpin({
verticalbuttons: true,
verticalupclass: 'material-icons touchspin-up',
verticaldownclass: 'material-icons touchspin-down',
buttondown_class: 'btn btn-touchspin js-touchspin',
buttonup_class: 'btn btn-touchspin js-touchspin',
min: parseInt($quantityInput.attr('min'), 10),
max: 1000000,
});
verticalbuttons: true,