You can style the map with the Google Maps API version 3 by using the parameter stylemap.
This should be a json variable. You can make it here.
In the overlay Map Style push the button at the bottom JSON and copy this value including [...].

The next example is for setting off the roads:

{mosmap styledmap='[{featureType: "road", stylers: [{ visibility: "off" }]}]'}


Other interesting possibilities:

Parent Category: Google Maps
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