Audio Pro Main Menu
Design and Skin Customization
You can do that with the Site Builder menu editor.
I think it's briefly covered in this video.
https://www.jamroom.net/video-training/80/customization-introduction-to-site-builder
#sb-site, .sb-site-container {
background-color: #cccccc;
min-height: 100vh;
position: relative;
width: 100%;
z-index: 1;
}