Hello, @Saira,
Actually, Those buttons are coming from a third-party plugin and the page on which the button appears is check out which is also coming from the WooCommerce plugin so the theme options will never work on those buttons you have to use the custom CSS code to make those buttons attractive as per your need. You can add the CSS code in the Dashboard >> XStore >> Theme Options >> Theme Custom CSS.
Regards 8Themes Team.