[SOLVED] Transparent header turns solid white upon scrolling

Hi Atahualpa community,

I need help on something that I haven't found the solution to. I have a header that includes a logo and a menu bar and is fixed in its position at the top. I would like the header to be transparent when I'm at the very top of the page so that the beautiful background image can show through, but turn solid white upon scrolling. I can't figure out what CSS code I should insert in order to accomplish this. Your help will be greatly appreciated.

Thank you.
Daniel