@nguyenmv2/buy-button/src/styles/embeds/sass/conditional.css

BuyButton.js allows merchants to build Shopify interfaces into any website

.shopify-buy__modal {
  display: none;
}

.is-active {
  & .shopify-buy__modal {
    display: block;
    opacity: 1;
    margin-left: auto;
    margin-right: auto;
  }
}