The Azure maps blog states Grayscale maps: A dark-gray map style is for users who want to have a darker scale representation of a map.
How can you enable this in the Javascript implementation?
You can set style: "grayscale_dark"
when you define the map.
var map = new atlas.Map("map", {
center: mapCenterPosition,
zoom: 15,
style: "grayscale_dark"
});
You can also just create a control on the map that allows you to switch between different modes:
map.addControl(new atlas.control.StyleControl(), { position: "top-right" });