Google map custom marker with css rounded corner

As of version 3.17, the google.maps.Marker objects exists in the markerLayer pane which is just a fancy name for a div.

To get a reference to the markerLayer you need to create an OverlayView Object. Now, this object is a bit abstract. You need to implement a draw function for it to work. For example, open the basic example in a new tab and paste this to the console

var overlay = new google.maps.OverlayView();
overlay.draw=function() {};

overlay.setMap(map);

overlay.getPanes();

it returns:

{
    floatPane: div
    floatShadow: div
    mapPane: div
    markerLayer: div
    overlayImage: div
    overlayLayer: div
    overlayMouseTarget: div
    overlayShadow: div
}

Thay markerLayer is a div which contains the markers. If I create your marker using a given icon image,

var marker = new google.maps.Marker({
                position: map.getCenter(),
                map: map,                          
                icon: 'http://ruralshores.com/assets/marker-icon.png',
                optimized:false
             });

My markerLayer will be:

Where the selected div (the one with z-index 103) is the markerLayer.

If you wanted to access the markerLayer programatically, you could add a "markerLayer" class to it after getting its reference with the getPanes method. I guess that every image inside the markerLayer is a marker, so you could style it at will.

TL/DR : you can style it, provided you went through all the trouble of finding the DOM reference to your marker.

Edit: I made a bl.ocks for you to check

When you know the url of the image used for the marker you know how to access it via CSS: use a attribute-selector.

Let's create a circle-marker based on your avatar with a 1px black border:

Marker-setup:

icon:{
       url: 'https://www.gravatar.com/avatar/0a9745ea7ac5c90d7acadb02ab1020cd?s=32&d=identicon&r=PG&f=1',
       //the size of the image is 32x32, 
       //when you want to add a border you must add 2*borderWidth to the size
       size:new google.maps.Size(34,34)},
       //define the shape
       shape:{coords:[17,17,18],type:'circle'},
       //set optimized to false otherwise the marker  will be rendered via canvas 
       //and is not accessible via CSS
       optimized:false
     }

the CSS:

  img[src="https://www.gravatar.com/avatar/0a9745ea7ac5c90d7acadb02ab1020cd?s=32&d=identicon&r=PG&f=1"]{
    border-radius:16px;
    border:1px solid #000 !important;
  }

....done.

Demo: http://jsfiddle.net/doktormolle/5raf237u/

When you use a shadow use a larger size(depending on the size of the shadow ):

http://jsfiddle.net/doktormolle/L2o2xwj3/