Transparent header to turn opaque with a border once users scrolls down

Hi Enfold
Thank you for a great theme. We use it on all our websites (separate licenses) and are super happy with it!
I have a question about the transparent header on https://metafora-studio-arts.org/
I would like this transparent header background to turn solid once scroll down starts, preferably with some kind of border and perhaps with the non transparent logo we use. On one of our other pages we managed to do it, but now I am not sure how we did it: https://metafora-arteterapia.org/ (here, I do not like the fact that the header scrolls down semi transparent, but the logo is solid)

I have browed though all our additional Css and all the teme options and I cannot find where I can edit this.