Changes WooCommerce Product Retailers buttons (Note: this example relies on FontAwesome being loaded already)

woocommerce-product-retailers-buttons.css

/*---------------------------------------
WooCommerce Product Retailers adjustments
---------------------------------------*/
.wc-product-retailers-wrap > p {
	text-align: right;
	font-weight: 700;
	font-size: 110%;
	margin: 15px 0 5px 0;
}

/* change opacity of all buttons on hover */
ul.wc-product-retailers a.button.alt:hover {
	opacity: 0.9;
}

/* Add Amazon colors / FontAwesome icon */
ul.wc-product-retailers a.button.alt.wc-product-retailer-amazon {
	background-color: #f3a847;
	padding-left: 10px;
}

a.wc-product-retailer-amazon:before {
	font-family: 'FontAwesome';
	font-weight: normal;
	font-style: normal;
	content: '\f270';
	display: inline-block;
	text-decoration: inherit;
	padding-right: 5px;
}

/* Add Apple colors / FontAwesome icon */
ul.wc-product-retailers a.button.alt.wc-product-retailer-apple {
	background-color: #333;
	padding-left: 10px;
}

a.wc-product-retailer-apple:before {
	font-family: 'FontAwesome';
	font-weight: normal;
	font-style: normal;
	content: '\f179';
	display: inline-block;
	text-decoration: inherit;
	padding-right: 5px;
}

Hi, I want to change the WooCommerce Product Retailers buttons, but which php should I put these code in?