How to change the header background color after scrolling down

Custom CSS is needed:

.is-sticky #sp-main-menu > .container > .row:first-child,
#sp-main-menu-sticky-wrapper,
.is-sticky #sp-main-menu > .container > .row:first-child::before,
.is-sticky #sp-main-menu {background: green;}

Change the value of the background color if you need a different color, you can use HEX or RGBA colors as well.