Reply To: Sticky Header Background Color – version 3.3.1

This was the CSS responsible for putting the transparent background in the sticky header:

.main-header, .sticky-header.header-clone, .header-spacing {
    background-color: transparent;
    background-repeat: repeat-x;
    background-position: center center;
}

I solved the problem by adding this to the child theme:

.sticky-header {
    background-color: white !important;
}