Header Opacity

Hello,

I’ve just downloaded the Ignis theme. The header photo is too dark for what I want. Is there a way to change the opacity of the overlay or to change the color? when I use the css code:
.wp-custom-header::after {
display: none;
}

it takes away the overlay completely, but the image background once the overlay is removed completely clashes with the moving text, making it hard to read.
How can I change the darkness or color of the overlay that’s over the background image?