Needing Code to turn transparent header opaque when mega menu is open

Hi,

I have a header that I want to be transparent most of the time, except when hovered over or when one of two mega menus is open. I have figured out the css for the hovering as below, but can't yet for the mega menu. The mega menu is from Will Myers. Please advise. 

Thanks!

Quote

header#header:hover {
  background-color: #ffffff !important;}