Either one would require some CSS to add the gradient overlay.
Add the background image to the Header and then add this CSS:
.site-header, .inside-header {
position: relative;
}
.site-header:before {
content: '';
position: absolute;
left: 0;
right: 0;
top: 0;
bottom: 0;
background: rgba(255, 255, 255,0);
background: linear-gradient(174deg, rgba(255, 255, 255, 0) 30%, rgba(0, 212, 255, 1) 100%);
}
Adjust the linear-gradient to suit.
This site is great for that:
https://cssgradient.io