I had a very difficult time finding this additional CSS that actually worked for me. In Appearance → Customize → Additional CSS
nav.fixed.scrolled {
background: rgb(249 247 245 / 0.7)!important;
}
My nav/menu background is normally white, the rgb above makes the sticky header a nice shade of off white, while the 0.7 transparency works very well.
This also worked, but pure white with transparency didn’t look as good:
nav.fixed.scrolled {
background: #0007 !important;
}
Note that I found this as a solution, but it did not work for me:
#header .is-sticky .top-header {
background-color: #00000026;
}