Hi

Thanks for contacting us. You can use this custom CSS

body .sppb-btn.sppb-btn-default, body .sppb-btn.sppb-btn-primary, body .sppb-btn.btn-primary, body .btn.sppb-btn-default, body .btn.sppb-btn-primary, body .btn.btn-primary{
    background-color: #861310 !important;
}

Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

-Regards.