How to make MARTA header/navigation NOT transparent?

Anyone know? I'd like to make the header not transparent, which it currently is. I'd like it to be a full opaque colour.

@magzkan Add to Home > Design > Custom CSS

/* Header Background - Entire Site - Desktop */
.Header {
   background: #ff0000 !important;
}
/* Header Background - One Page - Desktop */
body#collection-56323773e4b0b01121853d2f .Header {
   background: #ff0000 !important;
}
/* Header Background - Mobile */
.Mobile.loaded .Mobile-bar {
   background: #ff0000 !important;
}

Reference: https://beaverhero.com/marta-template-squarespace/#HeaderBackgroundColor