Styling the Main Menu screen is achieved using CSS! In your main.css file in the style folder, you'll want to insert something like:
[data-screen="main"]{
background-image: url("someURL");
}
or
main-screen {
background-image: url("someURL");
}
Make sure the URL that you are linking to is relative to main.css so for instance if you want to refer to something in your assets folder, you'd start with .. to go back a level, like so "../assets/ui/background.png" if you put the image in your ui folder.