You can only use Customizer to customize what has explicitly been defined by your theme to be customizable. But you can ALWAYS use code to make changes 😀
The CSS code below should make the header background for mobile the same as on desktop. This goes in the “Additional CSS” box of Customizer.
@media only screen and (max-width: 480px) {
.design-1.header-image .main-header {
background-color: #dbbfd8;
}
}
Thank you @gappiah !!! Got it finally