How do I keep header background black, when scrolling down?

Try adding this to your custom CSS (Appearance → Customize → Advanced Options → Custom CSS):

.sticky-enabled .tc-header {
   background-color: #000;
}

By the way, you’ve got a lot of weird stuff going on in your theme’s style.css file. You shouldn’t be editing the theme files directly. If you ever update the theme, you’re going to lose your changes.

That worked. Thanks so much for that mate.

Cheers.

Do you know how to do the same thing for the tablet/smartphone menu? When you click on the three bar menu icon, you get the menu with the same grey background.

Tablet example.

add this:

.nav-collapse.collapse, .no-navbar .nav-collapse {
    background: #000;
}

if you want to remove the rounded cornered add this:

.nav-collapse.collapse, .no-navbar .nav-collapse {
    background: #000;
    border-radius: 0;
}

Great. Thanks for that!