Hi.
GeoChart is built on Leaflet.js.
With API method chart.leaflet()
you can get leaflet map object to use advanced (Leaflet) settings.
So here is an example on how to do that:
http://jsfiddle.net/j93cL0gu/
Additionally to style some html elements, you can just use CSS rules:
.leaflet-bar a {
width:30px;
background-color:#09c;
color:#fff;
}
Here you can find Leaflet.js documentation:
http://leafletjs.com/reference-0.7.7.html