custom-css Customizing Site Nav on scroll

Site URL: https://level7tx.com/home

I'm wanting to change the navigation on scroll to a solid colored background. I have the navigation fixed at the top and want to keep it that way but to change the color of the background upon scrolling. The code I'm currently using is below. Any ideas on what code I need to use to change the color upon scroll?

header#header {
 position: fixed;
 padding: 0;
 height: 140px;
 background-color: transparent !important;
  -webkit-backdrop-filter: blur(3px);
  backdrop-filter: blur(3px);
}