Hello Johann,
In case you want to change the background color of the sub menu I would suggest you please add this custom CSS code to the Global CSS.
nav.x-nav-wrap.desktop .sub-menu {
background-color: #2196F3;
}
In case you want to change the font color of the submenu item I would suggest you please add this custom CSS code to global CSS.
nav.x-nav-wrap.desktop .sub-menu a {
color: #FF9800;
}
Please feel free to change the color code as per your design. The purpose of providing custom CSS is to show you how to add CSS code to your site. Writing custom CSS is outside the scope of our theme support. If you need more customization, you need to learn CSS and learn how to use the browser’s element inspector or you subscribe to One where customization questions are answered.
Hope it helps.
Thanks