How To: Change The Single Product Add To Cart Button Style

Please add this to Theme Options => Custom CSS:

.woocommerce form.cart button.add_to_cart_button,
.woocommerce form.cart button.single_add_to_cart_button {
color: #fff !important;
border-color: #ccc !important;
background-color: #ccc !important;
}

.woocommerce form.cart button.add_to_cart_button:hover,
.woocommerce form.cart button.single_add_to_cart_button:hover {
color: #fff !important;
border-color: #222 !important;
background-color: #222 !important;
}