To customize button in Quick View Summary Lightbox in Squarespace, you can use this code to Custom CSS box
div.lightbox-inner .sqs-add-to-cart-button {
background-color: #f1f !important;
border-color: #fff !important;
}
div.lightbox-inner .sqs-add-to-cart-button * {
color: #fff !important;
font-size: 14px !important;
}
Result