To remove the header overlay completely:
.header-image::after { display: none; }
To change its opacity:
.header-image::after { background-color: rgba(37, 46, 53, 0.9); /* Change 0.9 to the value you want */ }
INFO: This is a CSS snippet. If you don’t have a child theme or don’t know how to use one, we recommend that you add this snippet in Customize > Additional CSS.