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Open:

• theme.liquid
  OR

• base.css

Paste CSS before </head> (if using theme.liquid)

 <!————— Start of Buy Now Button Style Code —————>

<style> 

         .shopify-payment-button__button–unbranded {

         font-size: 0 !important;

         background-color: #FFD72D !important;

         color: #000 !important;

            border: solid 2px black;

         }

         .shopify-payment-button__button–unbranded::before {

         content: ‘ORDER CASH ON DELIVERY’ !important;

         position: initial;    

         visibility: visible;

         top: 0;

         left: 0;

         bottom: 0;

         font-size: 16px;

            font-weight: 600;

         display: contents;

         }

        </style>

<!————— End of Buy Now Button Style Code —————>