Hello, @Muhammad Shoaib,
Please copy and paste the below-given CSS code to XStore >> Theme Settings >> Theme Custom CSS >> Global CSS and check back to your site after removing the cache.
.et-mini-content .woocommerce-mini-cart__footer-wrapper p.buttons.mini-cart-buttons a.button.btn-checkout.wc-forward {
background-color: green !important;
}
div#payment button#place_order {
background-color: green !important;
}
Regards 8Themes Team.