My!!!, you are right,
[giving an element a transparent-white background, when its positioned over/inside an element that also has a solid-white background, will not make a jot of visual difference. For example, your .site-header element has a background-color of solid-white, and the .branding-wrapper element (that you’re trying to make transparent-white) sits inside/over it, so you’ll need to target both in your efforts for overall transparency. Maybe make .site-header completely transparent before you try to give .branding-wrapper a semi-transparency.]
I am going to dig that part right away!