Remove bigcommerce cornerstone theme unwanted header and footer space frequently asked question at bigcommerce community forum.

copy and paste below CSS at footer script

<style>
    .header-logo {
        margin: 0rem auto 0rem; 
    }
    .footer { 
        padding: 1rem 0;
    }
    .footer-payment-icons {
        margin-top: 0rem;
    }
    .footer-info-col { 
        margin-bottom:1rem; 
    }
</style>