Hi

Thanks for contacting us. Use this custom CSS

.bg-gradient-primary{
    background: linear-gradient(255.48deg, rgba(255, 255, 255, 0.2) 4.07%, rgba(255, 255, 255, 0) 103.07%), #0fbce3 !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.