change scroll length before header menu changes opacity?

good evening,

i’m enjoying the Septera theme, but wonder if there is a way to adjust how “soon” the header menu changes from transparent to opaque when scrolling down the page?

to clarify :
when my page loads (on regular wordpress mode, not landing page mode) the header image is completely visible, the header menu (which contains the blog title on the left, and menu pages on the right) is completely transparent. when the screen scrolls down, the header menu transitions to white.

where do i adjust how much “scrolling” happens before this transition? it looks to be the same height as the header menu (85px) but adjusting that value, or any other values i have found only moves the header menu or increases its size. nothing changes the scrolling “length” prior to the transition.

thanks.