Unoptimized markers exist as img elements inside the markerLayer mapPane.
(If your markers didn't had the optimized:false property, they will render inside a canvas element and won't be CCS stylable)
To get a reference to the markerLayer mapPane
var myoverlay = new google.maps.OverlayView();
myoverlay.draw = function () {
//this assigns an id to the markerlayer Pane, so it can be referenced by CSS