Add black border & hovering colour around a solid button

Add this Custom CSS to your site:

.ProductItem-details .sqs-add-to-cart-button {
  font-size:12px;
}
.tweak-product-item-details-add-to-cart-button-style-solid .ProductItem-details .sqs-add-to-cart-button {
  border:2px solid #000;
  background-color: #fff;
  transition:ease-in-out 300ms;
  &:hover {
    background-color:#000;
    color:#fff;
  }
}

Give me a thumbs up if this helped!