Hi Eno,
Try adding this through Appearance > Customize > Additional CSS:
header.site-header {
background-color: var(--accent);
background-image: linear-gradient(180deg, rgba(255, 255, 255, 0.1), var(--contrast-3));
box-shadow: 2px 10px 10px -5px var(--contrast);
opacity: 1;
mix-blend-mode: multiply;
}