Hello,
If you want to increase the transparency of the header under scroll down, please add the following css inside your child theme’s style.css file or Additional css section inside customizer;
.et-fixed-header#main-header {
opacity: 0.33;
}
Note: Adjust the value of opacity of your own between 0 to 1.
Hope this is what you are looking for. Please let us know
Thanks!
Sushil