Changing eader image opacity

Boy, what a shocker, i enjoy this Theme a lot, but, where on Earth did you insert or place this header CSS
"<style type=“text/css”>
.site-header:after {
content : “”;
display: block;
position: absolute;
top: 0;
left: 0;
background: url( no-repeat;
background-position: center top;
background-attachment: fixed;
width: 100%;
height: 100%;
opacity : 0.4;

I need to change that awful opacity for the header image please, where do i do this??
My site is

In the custom-header.php file.

What code should i modify in the custom-header.php?

You shouldn’t modify anything in that file, you just need to add this to your custom CSS plugin:

.site-header:after {
    opacity: 0.4;

Change 0.4 to the value you want.