Change color of the Navigation Bar / Sidebar in the modern player

Technically, you can change it with the code below:
const sidebar = document.getElementById('sidebar');
sidebar.style.backgroundColor = "#f2f2f2";

But in this case you would have change the appearance of all list menu items with its states (such as hover, viewed, selected) including the font color to match your new  background color. Anyway, this code will indeed change the background color of the sidebar only.