I have loaded kml file showing country names on globe. How can I change the font size/ color?
There’s no way to set the default font/color for KML before load (though we may add it in the future). The way to do it for now is to iterate the entities after load and set the entity.label properties to your liking. An example of modifying entities after load is in GeoJSON Sandcastle example 3: http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=GeoJSON%20and%20TopoJSON.html&label=Showcases. All of data sources (including KML) work the same way, you just use KmlDataSource instead of GeoJsonDataSource.