To change color of subnav dropdown on a single page, you can follow these steps.
#1. Find Page ID.
In my example, we will have ID: #collection-6673f2e18432c25013aee99f
#2. Next, use this code to Custom CSS box
#collection-6673f2e18432c25013aee99f div.header-nav-folder-content {
background-color: #f1f !important;
}
#3. Result