Hi there,

#1. To change header color and add background, please use css code:

.header-2 {
    background-color: #your-color-code !important;
background-image: url('your-link-image-background');
    background-repeat: no-repeat;
    background-size: cover;

Paste into file style.css on child-theme.

#2. I added Mega menu for you, please check.
To edit menu, please follow guide at https://wpdocs.gitbook.io/freshio/menu-settings/mega-menu

#3. Maybe, you need to edit on Elementor Widget or css style.
Let me know where do you want to change color, I will check and help you