HI @helmaT,
Thanks for writing in!
Happy New Year to you too!
The best way to do is to add a background when the menu is sticky. Please add this CSS to your theme option -> CSS and let us know if that works for you.
.x-navbar.x-navbar-fixed-top.x-container.max.width {
width: 100% !important;
max-width: 100% !important;
background: url(//leveninnu.nl/wp-content/uploads/2018/12/kiki_canvas_light2-e1544184117613.jpg);
}
Hope this helps!
If you would like to learn CSS, I am sharing few resources that you take a look to get started with CSS and an interesting tool that you can use to speed up the development process.
I recommend you to watch following video that will help you to get started with CSS.
https://www.youtube.com/watch?v=MFR4WXiLzpc
Sometimes it can get a bit difficult to find out the right selector to be able to write the required CSS codes. A handy tool that can help you in this is Google Chrome dev tools. I am sharing the resource that you can refer to get started with dev tools.
https://developers.google.com/web/tools/chrome-devtools/css/
https://developers.google.com/web/tools/chrome-devtools/
https://www.youtube.com/watch?v=tP_kXBJWPhQ&t=200s
Thanks.